As has previously been pointed out in these pages the greater proportion and the larger variety of the explosives that are annually produced are consumed in the industries and a very considerable proportion of these are consumed in the winning of coal. As is well known this most important industry is attended by many hazards, not the least of which is the constant danger of explosions owing to the presence of fire-damp and inflammable dust in these mines. Most serious accidents from these causes which have been attended with frightful casualties have frequently occurred, and their frequency and magnitude have increased as the demand for coal has increased until the public conscience has been aroused and efforts have been made by individuals and by governments to devise means by which to reduce the number of these explosions and limit their scope.
Consideration of the conditions attending such of these catastrophes as were carefully investigated made it evident that many of these mine explosions have been initiated by the explosives used in the mines, and therefore the behavior of a large variety of explosive compositions, when fired in dusty and fiery atmospheres, have been studied experimentally with a view to selecting from among them those which, while capable of doing the work required of them efficiently at a reasonable cost, and while possessing such qualities as to render them reasonably safe in transportation, storage and use, were least liable to ignite the firedamp, or coal-dust-air mixture, or mine-gas-coal-dust-air mixture found in mines.
For this purpose it became necessary to have a chamber in which the gas and dust could be introduced, and the explosive fired, at will, and all the conditions of the experimental trials be known and under control. Beginning some 30 years ago, many kinds of chambers have been employed, from one made of boiler iron mounted on wheels used at Zwickau, Germany; an abandoned mine tunnel used at Rossitz, Austria; a wooden gallery used at Frameries, Belgium; a concrete gallery used at Lievin, France; to metal galleries as used at Woolwich, England, and Pittsburg, U. S. This last is one of the most modern of these testing galleries, it having been erected on the arsenal grounds by the Technologic Branch of the U. S. Geological Survey in 1908, and as it was designed after careful study of the characteristics of different galleries abroad it may be regarded as representing the latest type of testing chamber.
This chamber, which is styled Gas and Dust Gallery No. 1, is shown in Fig. 1. It consists of a cylinder 100 feet in length and 6 1/3 feet in diameter, which is built of boiler-plate steel, in five divisions, each consisting of three sections, 6 2/3 feet long. The gallery is closed at one end by a concrete head. The different sections of the gallery are for convenience in operation, numbered consecutively from 1 to 15, beginning with the section nearest the concrete head. Sections numbered 1, 2 and 3 are made of 1/2- inch plates, the remaining sections of 3/8-inch plates, and all of steel having a tensile strength of not less than 55,000 pounds to the square inch. The sections are held together by lap joints, at each of which there is on the interior of the gallery a ring, formed of 2 1/2-inch angle iron, upon the face of which paper diaphragms may be so secured as to partition off any desired portion of the gallery at will, and thus provide a closed space of any desired volume, within the capacity of the gallery, in which to enclose the gas-air, coal-dust-air, or gas-coal-dust-air mixture to be used in the test.
Each section is provided with a pressure-release door placed centrally on top, which not only provides a vent by which the gases may immediately escape after the explosion, and thus acts as a safety valve to prevent the destruction of the gallery; but also affords an approximate means of estimating the pressures developed.
Each door closes on a rubber gasket and is provided with a rubber bumper on its back to prevent injury when thrown open violently. In use each door may be left open, or closed but not
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FIG. 1.- Gas and dust gallery, No. 1, Pittsburg Testing Station.
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FIG. 2.-Barricades at Mouth of Gallery.
fastened, or closed and fastened, as seems best under the experimental conditions which obtain.
Each section is provided with a stout plate-glass window placed in the center of the section on the operating side of the gallery through which the progress of any flame produced in the gallery may be viewed and noted, while an indicator cock, tapped into the central section, provides a means by which samples of the mixtures in the gallery may be taken for analysis.
The gallery is so connected with the natural-gas supply used in Pittsburg that it may be filled with gas at will, and the quantity charged is measured by a meter which can be read to 1-20 of a cubic foot. The air and gas are mixed by means of circulating systems exterior to the gallery, operated by Monogram exhausters. The circulating system for the first division is stationary and includes steam-heating coils by which to bring the mixture in the gallery to a constant temperature. The remaining divisions of the gallery are served by a portable device.
When coal-dust is to be used, it is spread on a series of shelves 20 feet long by 4 inches in width lining the gallery, there being four of these on each side, and in addition upon a steel trestle, having a surface 20 feet by 12 inches, which is placed for that purpose in the first section of the gallery when coal-dust is to be used. This dust is always freshly ground from lump coal to 100 mesh in fineness just before using.
In addition the gallery is provided with a humidifying apparatus provided with a Koerting exhauster, having a capacity of 240,000 cubic feet of free air per hour, by which the effect of moisture in preventing the propagation of explosions may be quantitatively ascertained.
The explosives to be tested may be suspended in the chamber and fired in the prepared atmosphere, and this method has been pursued at some stations, but the regular practice at the Pittsburg Testing Station is to fire the charge in a special "cannon," as this more nearly simulates the conditions in mining where the charge is fired in a bore-hole in coal or rock. These "cannon" are cylinders 24 inches in diameter by 36 inches in length, with bore-holes 2 1/4 inches in diameter by 21 1/2 inches in depth. The simplest of them have been made in one piece from a low carbon-steel, or nickel-steel, forging. Others have been built up from centrally perforated jackets of cast steel, vanadium steel or other iron alloy, and a liner of nickel steel or other metals or alloys. In repairing, after erosion, the liner has been formed by the Thermite process. No definite conclusions have yet been reached as to the relative merits of the different forms of construction. These " cannon " are shown in the right foreground of Figs. 2 and 4 and in the center of Fig. 6.
The " cannon " is imbedded in the concrete head of the gallery and is so laid that its axial line coincides with the axial line of the gallery. The " cannon " is loaded from within the gallery, but the charges are fired by electric detonators for high explosives and electric igniters for explosives of the gunpowder class, the firing machine being located in an observation room 6o feet distant from the gallery. The larger part of the explosives tested are detonated, and they are fired both stemmed and unstemmed into the sensitive mixtures.
As, when the explosive mixtures in the gallery are fired, the blast from the mouth of the gallery is very destructive in its effects, two concrete barricades are erected on each side of the mouth of the gallery, and a thick iron plate is so suspended on a frame across, but at a distance of so feet from, the mouth of the gallery that it may so swing as to deflect the blast and arrest any flying stemming or other material which may be blown out of the gallery. This arrangement is shown in Fig. 2, which also shows the reinforced-concrete foundation in which the gallery is set and specimens of the " cannon " used. The violence of the gallery explosions initiated by the charges of explosive used may be judged from Fig. 3, which is a photograph of an explosion of a coal-dust-air mixture.
The observation room from which the charge is fired, and the visible phenomena occurring in the gallery observed, is shown in Fig. 4. This room is 40 feet long by 9 feet 5 inches wide, built with brick walls 18 inches thick and provided with a heavy plate glass window, 37 feet long by 6 inches wide, which is protected by two projecting wooden guards. This room with its window is made so large in size that it may not only provide a large field of sight, but that it may accommodate a considerable number of persons, for the station is designed to be educational as well as experimental, and coal miners are brought there in large numbers to be convinced by experimental demonstrations of the accidents that may arise unless they use the explosives recommended by the station, and use them in the prescribed manner and amount.
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FIG. 3.-EXPLOSION OF A MIXTURE OF COAL DUST AND AIR.
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FIG. 4.-Observation Room.
Prior to testing the explosives in the gallery, which is both costly and time consuming, they are subjected to other tests which may show them to possess such characteristics as to render the munfit for use and render the gallery test unnecessary. Thus they are inspected physically and analyzed chemically, and an admirably equipped and well-manned laboratory is provided for this purpose. Among other tests the gravimetric density of the material, in the original package in which it would be used in the mine, is determined by the aid of dry sand as the mobile medium, and this density is carefully preserved in those tests in which density is a factor.
One of the first tests to be made is the determination of what is styled the "Unit-Disruptive Charge," which is ascertained by the aid of the ballistic pendulum shown in Fig. 5. This apparatus consists essentially of two parts; the "cannon," in which the charge is fired, and the pendulum, which receives the impact of the products of the explosion, and that of the stemming when the latter is used. The " cannon " is identical in form, construction and proportions with those used in the gallery. It is mounted on a four-wheeled truck, to which it is made fast by straps and rods. The truck runs on a 30-inch track which is provided with a recoil bumper placed 9 feet from the face of the pendulum mortar. The " cannon " is carefully placed axially in line with the pendulum mortar.
The pendulum consists of a 12.2-inch army mortar, weighing 31,600 pounds, which rests in a stirrup made of two 1 1/2-inch machine-steel rods bent in a U-shape. The ends of these rods pass through a solid steel supporting beam and are held fast to it by cast-steel saddles fitting over the beam. This beam is provided with two nickel-steel knife edges countersunk in its lower face, which rests on bearing plates provided with small grooves that permit of the knife edges being kept in oil, so as to be protected from the weather. The bearing plates rest on base plates which are anchored to the concrete piers between which the pendulum mortar swings.
The concrete piers are each 51 by 120 inches in dimensions at their bases and 139 inches high. The outside walls taper while the inside walls are vertical, and there is a clearance of 60 inches between these piers. A firing line, with a coupling box, is attached to the left-hand pier and by its aid the firing may be done from a safe distance.
The extent to which the pendulum mortar is deflected is measured by a detachable device consisting of a graduated scale with its vernier, which are set on a steel base fastened to a concrete footing below and to the rear of the mortar, the movable parts being actuated by a contact rod, set in guides, which bears at one end against a stud-bolt in the bottom of the mortar directly below the point at which its center of gravity is located, and, at the other, on the scale. The radius of the swing of the pendulum mortar measured from the knife-edge bearings to the center of the contact rod, or to the base of the stud-bolt, is 114 7-16 inches. The radius of the swing measured from the knife-edge bearings to the center of the trunnions of the mortar is 89 3/4 inches. The recording device measures the deflection of the pendulum to within the 1-100 part of an inch.
The standard used for this test is 227 grams (1/2 pound) of 40 per cent straight dynamite, stemmed with 1 pound of dry clay, tamped with tamping sticks of standard pattern under a uniform rate of pressure and fired with a No. 6 electric detonator. The unit-disruptive charge of another explosive is that weight of this explosive that will, when fired under the prescribed conditions, give the same deflection of the pendulum mortar as the standard dynamite charge does.
In making the test after the recording device is set, the " cannon " is loaded in the prescribed manner and rolled up to within 1-16 of an inch of the muzzle of the pendulum mortar and stops are so placed that this distance is maintained when the flanges of the wheels of the truck are against them. The legs of the detonator are then connected to the firing line and the party loading, who has also been carrying the safety plug with him, retires to the firing machine, inserts the safety plug and fires.
Firing trials with the standard show that variables enter here and-that the same pendulum deflection is not invariably obtained with equal weights of charge of the standard dynamite, even though the successive charges are tamped in the " cannon " with the same degree of pressure, stemmed with the same weight of fire-clay and fired by the same numbered detonator. A condition affecting this result is the distance of the " cannon " from the mortar, and hence, as shown above, this distance has been definitely fixed. Another is the position of the knife edges supporting the pendulum mortar, and to eliminate the effect of this the knife {picture}
FIG. 5.-BALLISTIC PENDULUM
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FIG. 6.-TRAUZL TEST.
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FIG. 7 .—SMALL LEND BLOCK TEST.
edges are trammeled before each trial. Other factors affecting the results are the direction and velocity of the wind, the condition of the bore-hole in the "cannon," and that of the person charging the "cannon." No one of these can be controlled absolutely.
Hence the explosive to be tested must be proved in this ballistic-pendulum test directly against the standard, both being handled and charged by the same person on the same day, and as nearly as possible under the same conditions, for this tends to eliminate the personal equation, through its effects becoming nearly uniform, and to eliminate the effects of wind and weather, since they are fairly uniform during the trial periods. The condition of the bore-hole is the existing variable factor which is the most difficult of elimination.
The energy of the explosive is further ascertained comparatively by the Trauzl and by the small lead-block tests. The device used in the Trauzl test is shown in Fig. 6, which consists of lead blocks 200 millimeters in diameter and 200 millimeters in height, in which holes 25 millimeters in diameter and 125 millimeters in depth are bored centrally in the top. These cylinders are cast from desilverized lead and all used in the same series of tests are carefully prepared under identical conditions and from the same melt. The volumes of these bore-holes are carefully measured by means of water.
Ten grams of the explosive, weighed on a balance of precision, are wrapped in similar-sized pieces of tin foil, together with a No. 7 electric detonator, and the whole inserted in the bore-hole of one of these cylinders, then 40 cubic centimeters of dry Michigan dune sand, of such fineness that it will all pass through a 50-mesh screen and be caught on an 80-mesh one, are poured into the bore-hole and tamped ten blows with an automatic-tamping device, which operates on the principle of an automatic-center punch, and delivers blows of known magnitude over a definite area. Then 10 cubic centimeters more of the same sand are poured in and tamped with forty blows of the tamping device. The loaded cylinder is placed on a piece of heavy shafting imbedded in concrete, which forms a rigid support, the temperature of the block is ascertained to make sure that it is 15° C., and, when this is attained, the charge is fired. The cavity is again measured by calibration with water and the increase in volume produced by the charge of explosive fired is compared with that produced by an equal weight of the standard dynamite fired under the same conditions. The Trauzl test is quite widely used, and abroad it has been recommended that the cylinder be covered by a lead plate secured by a yoke. But it is found that in putting on the yoke the charge and stemming were disturbed and, as uniformity in these last particulars are of much more importance in these comparative tests than a greater degree of confinement, the plate and yoke are not recommended.
The Trauzl test measures comparatively the displacing effect of an explosive under moderate confinement. The small lead block test measures comparatively the pressure exerted in contact by a charge of explosive which is detonated or exploded unconfined, or, in other words, the percussive effect. The blocks used, and the deformations produced on some of them are shown in Fig. 7. The lead blocks employed are cylinders 132 inches in diameter and 272 inches high. The charge of explosive used is 100 grams. Since an unconfined charge of high explosive, such as the standard dynamite, would, on detonation in contact, deform the cylinder beyond measurement, as shown by F in the figure, a disk of annealed steel 1 1/2 inches in diameter and 34 inch in thickness is placed upon the lead cylinder. Since this plate retains a portion of the energy expended the compressed cylinders record only the residual energy.
In making the test, after placing the steel disk on the cylinder, a strip of manilla paper is so secured about and beyond them as to provide a container, above the plate, for the explosive to be fired. The cylinder is then placed on a rigid support, the carefully weighed charge of explosive poured in and so tamped as to acquire the specific gravity it possessed in its original container, the No. 7 electric detonator inserted, and the charge fired. The height of the compressed cylinder is then ascertained with precision, and this comparison, as compared with that produced in a cylinder subjected to the detonation of 100 grams of the standard dynamite, is styled the relative percussive force of the explosive.
A more accurate idea of the pressures produced by an explosive may be obtained by means of the Bichel pressure gage which is employed to determine the "maximum pressure of an explosive in its own volume," by which is meant the maximum pressure which an explosive exerts when exploded or detonated in a space which it fills completely, and when all of the heat set free through
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FIG. 8.-Bichel’s Pressure Gages.
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FIG. 9.-METTEGANGS’S EXPLOSION CALORIMETER
the chemical reactions taking place during the explosion are retained by the products of the reaction. Evidently this condition never actually obtains in practice, for a portion of this heat is always communicated to the walls of the enclosure. The portion thus lost from the products differs in amount with the temperature differences, with the rate at which the heat is set free in the reaction, with the materials of which the chambers are composed and the extent of the exposed surface in the chambers. Using chambers made of the same material it becomes possible, by varying the areas of these exposed surfaces in the different experiments to a known extent, to measure the heat-absorbing effect of surfaces of known area, and by combining this data with that recorded for the pressure, to ascertain the total dynamic effect of the charge of the explosive tested. This apparatus moreover affords a means by which the gaseous, liquid and solid products of the reactions attending explosion may be collected for chemical analysis and physical inspection and study.
In Fig. 8 are shown two of these gages, one of which is open and ready to load while the other is closed for firing. It will be observed that they are in principle like the bombs used by Noble and Abel in their Researches on Fired Gunpowder, but they are markedly developed in details of construction and in the addition of accessories. One of these cylinders has a chamber capacity of 15 liters, the other of 20 liters, but the volume of the latter may be reduced at will by inserting steel disks of known volume and area. The surface is thereby changed so that while the cooling surface of the small chamber is 3914 square centimeters, that of the large cylinder, when one large steel disk is enclosed, is 6555, and, when three of the smaller disks are enclosed, 7624 square centimeters.
These cylinders are made of steel 12.5 centimeters in thickness and the removable heads are secured in place by 12 heavy studbolts, packed with lead washers, and an iron yoke. A system of sheaves and suspended counter-weights is provided to aid in detaching the heavy heads from the cylinders and mounting them upon the specially designed wagons, so as to give ready access to the interiors of the cylinders.
Before the charge is fired the cylinders are exhausted to 10 millimeters of mercury. To permit of this being done a well glanded tube is inserted in a perforation in an upper segment of each cylinder near one end, and this is provided with a valve by which to isolate the air-pump after the vacuum has been attained. On the opposite upper segment of the cylinder from the exhaust a second opening is provided in which a glanded housing is inserted which affords a means for introducing the electric detonator and so packing its legs as to prevent air entering about them while the cylinder is exhausted, or gases escaping about them after the explosive has been fired and while a considerable pressure obtains within the cylinder.
A third perforation in the top of the segment carries a properly glanded tube provided with a piston 0.3937 centimeters in diameter which can move up and down within this tube. This piston is held on its seat by springs of known dimensions and effect. A stylus is mounted on the upper end of the stem of the piston in such manner that it moves freely only in the vertical plane, while the motor-driven drum at its rear, against which it impinges, rotates horizontally, thus producing a curve which, by its magnitudes and variations, records the extent of the pressures developed from the beginning to the end of the explosion. The charge of explosive used varies from 100 to 200 grams according to its character, as judged by the results of the chemical analysis which has been made of it. The explosive is removed from its original wrapper and enclosed in a wrapper of tin foil in such manner as to preserve its original density. The head of the cylinder is removed, the No. 7 electric detonator passed through the glanded plug and then inserted and secured in the cartridge, the fused cartridge laid on a small wire support in the center of the cylinder, the head replaced, the vacuum produced, the indicator drum set in revolution, and then, all parts being found secure and operative, the charge is fired and the indicator diagram taken. Three shots are made with each of the different cooling surfaces.
After the explosion the products are allowed to cool to the room temperature which, with the tension of the gases, the barometer and the volume of the cylinder, is noted and the volume of the gaseous and vaporous products is reduced to normal. A sample of these gases and vapors is then drawn out through the exhaust opening and analyzed. The liquid and solid products are recovered, measured and analyzed after the head has been removed.
As is well recognized the heat developed by an explosive when it explodes is one of the most important of the factors which determine its effect, and since Berthelot first employed the calorimetric bomb with which to directly measure the number of heat units set free by a known weight of an explosive, attention has been given to the improvement of this device so as to render it more useful. One of the modern forms of this instrument is Mettegang's explosion calorimeter which is shown in Fig. 9.
This consists of a bottle-shaped calorimetric bomb 30 inches high, with a capacity of 30 liters, made of wrought steel 1/2 inch in thickness, and closed by a cap having an air-tight fit. Two holes are tapped into this bomb on opposite sides of the curvature of the neck, into one of which a valve is tapped by which to connect the bomb with an air-pump, and into the other a plug through which the legs of the detonator are carried. This bomb, when charged, is placed in an immersion vessel, filled with a known weight of water, which is made of nickel-plated copper 1-16 inch in thickness, and which is strengthened by bands of copper wire wound about the outside. The immersion vessel is placed, with its contained bomb, in a wooden-insulating vessel provided with a wooden cover. The accessories consist of a framework stirring device, rotated or raised by an electric motor, with which to bring the entire mass of water to a uniform temperature, a thermometer with open scale reading to 1-00 of a centigrade degree, a magnifying glass with which to read this thermometer, a pair of scales on which to accurately weigh the several parts of the system and the water that is used, a hocking frame to raise the vessels from the scales and deposit them in place, and a machine with which to fire the charge.
The water equivalent of the calorimeter having been determined and the effect of the detonator and tin-foil wrapping having been ascertained, a charge of 100 grams of the explosive, wrapped in tin foil, is connected with a No. 7 electric detonator and suspended in the bomb, which is closed and exhausted down to 10 millimeters of mercury. The immersion vessel having received its weighed charge of water, all the parts of the calorimeter are assembled and the stirring device set in motion to bring the water, and therefore all essential parts of the calorimeter, to a common temperature. When the thermometer, immersed in the water, shows that a constant temperature has been reached the charge is fired and the rise in temperature recorded on the thermometer carefully observed until the mercury has reached its greatest altitude in this experiment. From this data, together with that referred to above, the number of calories given by a known weight of the explosive is found.
In firing by detonation it is essential for safety and success is blasting that when the reaction is once initiated by the detonator it shall proceed throughout the column of explosive, for, otherwise, a portion of the charge may be thrown out of the bore-hole unexploded but inflamed, producing a "blown-out shot" which may ignite the fire-damp or coal-dust-air mixture, or a portion may be left in the bore-hole unexploded where it constitutes a source of danger in subsequent operations, or it may be brought down mixed with the coal to produce trouble in the breakers, in transportation, or in the use of the coal. The test employed to ascertain the relative sensitiveness of explosives to the detonation of masses of their own kind is called the explosion by influence test. I have applied this term to the testing of explosives for their sensitiveness to the initial detonation of a standard explosive, and described the method in the journal of the American Chemical Society 15, 10-18; 1893. A commercial method now used for sometime is one in which the cartridges are placed in rows on the ground, or other support, with spaces between each, and varying the intervals between the cartridges in successive trials. A more modern and severer test is to so bind two cartridges together with wire that they may be suspended vertically in the air, end to end, at a carefully measured distance apart, a detonator, of the grade recommended by the manufacture of the explosive, being inserted in the lower end of the lower cartridge, secured in place, and fired. By proceeding tentatively the distance at which detonation by influence ceases is said to be established within very narrow limits. For comparison between different explosives cartridges of uniform size are used, such as i34 inches in diameter by 8 inches in length. This may necessitate the repacking of the explosive, and when this is done care should be taken to preserve the original density, since this is a factor in this behavior of explosive.
While the explosive should be sufficiently susceptible to detonation to be fired with certainty it should not be so sensitive to percussion as to be dangerous in handling, transportation, or use. This sensitiveness of explosives to the effect of direct blows is determined by the impact machine shown in Fig. 10, which consists of a vertical steel frame work carrying two guide rods between which a yoke, to which the impact weight or hammer is attached, is guided; an anvil upon which the charge of explosive is placed; and a soft-steel plunger which rests upon the anvil and upon which the hammer impinges in its fall. The yoke is provided with jaws which engage the lugs of an endless chain moving behind it, and by this mechanism the yoke, with its attached weight, is raised to any desired height. By means of an electric current the yoke may be magnetized and demagnetized at will, so that, when magnetized, it will attract and support the hammer to such an extent that both may be raised together by the endless chain to any predetermined height, at which point the yoke is demagnetized, the hammer, or weight, is released, and the latter then falls through the intervening distance and impinges upon the plunger. The stop which arrests the upward travel of the magnetized yoke and automatically causes its demagnetization is operated by a vertically driven precision screw on the right-hand side of the frame, which is also geared to a recording device which measures the height from which the hammer falls. By the aid of this screw and its accessories one is able to set the stop in advance, so as to secure any desired distance of fall for the hammer within the capacity of the machine.
The hard-steel anvil is set firmly on a heavy iron base and it is surrounded by a tubulated jacket through which water may be circulated so as to bring the temperature of the anvil, and of the charge of explosive placed upon it to any desired temperature and to keep it there. The plunger is held lightly in place by a steel guide which forms a part of the base support for the vertical guide rods. It is essential that the faces of the plunger and of the anvil which are in contact should be absolutely true and plane. As the impacts and explosions produce deformation of the metal, the plunger is made of soft steel so that the deformation may accumulate on it, and therefore it is very frequently turned up on a lathe and reground to true up its face.
The hammer used weighs 2000 grams. The soft-steel plunger weighs goo grams, and the maximum height from which the hammer can be dropped is 100 centimeters. The weight of charge of explosive used is 0.02 grams. The temperature of the anvil and the explosive at the time of testing is 25° C.
In making the test the explosive is weighed out on a chemical balance and the charge so wrapped in tin foil as to make a pellet in the form of a flat disk i centimeter in diameter. The hammer is raised, the stamp is lifted, the pellet is placed on the anvil, the stamp is pressed gently down upon it so as to insure a good contact, and the whole is left to attain the standard temperature. The stop is then set by judgment and the hammer raised until it is disengaged at the chosen height and falls upon the plunger. If no explosion ensues, the stop is set at a greater height and the hammer released, and this method of procedure is repeated until either explosion occurs on impact or the maximum range of the machine is reached. When explosion does occur the test is repeated with a fresh charge of explosive and slightly diminished distance of fall, and one thus proceeds tentatively until such a height of fall for the hammer is 'reached that there is no explosion, and yet if that height be exceeded by but i centimeter an explosion occurs. This point is then fixed by four additional tests giving the same results.
Provided all other conditions remain the same the brisant or shattering effect of an explosive varies with the velocity with which the chemical reaction, or explosion wave, travels through the column or charge of the explosive. Where explosives are fired by detonation this movement, as measured in definite terms of time and length, is styled the rate of detonation of the explosive. The making of such determinations is not new for Abel
measured the rate of detonation in gun-cotton, nitroglycerin and dynamite nearly forty years ago,* and Berthelot did so some ten years later.v What has been done in recent years has been rather in the standardizing of the method, the improvement in the details and operation of the chronograph, and the introduction of the method into general practice.
To assure a definite and uniform area of exposure, the cartridges of explosives in their original wrappers, but with the ends cut off so as to avoid the damping effect of the layers of paper, are packed in tubes of thin sheet iron 42 inches in length and varying in diameter from 1 1/2 to 2 inches, according to the character of the explosive to be tested. When the tube has been charged two copper wires are inserted through perforations in the
* Phil. Trans. 164. 377; 1874.
v Am. chim. phys. (6) 6, 556.
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FIG. 10.-IMPACT MACHINE.
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FIG. 11.-METTEGANG RECORDER.
tube and the cartridge file at a distance of i meter apart, and these wires are separately led to a chronograph. An electric detonator of the usual type and grade is inserted in one of the end cartridges of the file; the tube, as now arranged, is suspended in the firing chamber; the copper wires which pass through the explosive are connected up to the chronograph; and the charge is fired.
The chronograph which records the time that elapses between the rupturing of the wire nearest the detonator and the wire meter distant from it is known as a Mettegang recorder, and is shown in Fig. 11. The primary components of the Mettegang recorder are a soot-covered bronze drum so connected to an electric motor that it may be caused to revolve at any desired speed up to 105 revolutions per second; a 200-volt D. C. electric motor provided with a rheostat for controlling its speed; a vibration tachometer so connected to the bronze drum that the number of revolutions of the latter in unit of time are accurately measured for any speed between 50 and 105 revolutions per second; induction coils which may receive their electric current from electric lighting circuits having terminal pressures of from about 110 to 220 volts; and platinum terminals placed about 1/4 millimeter from the surface of the rotating drum, and in circuit with the induction coils, by which electric sparks are so projected against the surface of the drum as to disturb its sooty covering and produce a tiny bright spot at the point of impact, which spot may be easily perceived by the aid of a microscope attached to the drum.
The drum is 500 millimeters in circumference. The edge of this drum is provided with 500 teeth which may be made to engage an endless screw. A pointer attached to this screw passes over a dial reading to hundredths, and it thus enables one to read the distance intervening between the spots produced on the soot-covered surface of the drum with great precision. The drum is provided with six platinum terminals which are held by an insulated arm that may be so moved as to bring the points within any desired distance from the drum, and each one of these points may be put in series with one of the induction coils while the other end of the electric lead is grounded to the drum through the base which supports it. Only two of these platinum terminals are used in any single-firing trial for the determination of the rate of detonation in a given explosive, while the other four are held in reserve for future use.
To operate this method of ascertaining the rate at which detonation when once initiated is transmitted through a column, or file, of an explosive, the electric current which is used as the medium for transmitting the record is, as taken from its source, divided into two parts by passing it through two equal lamp resistances, each of which, at the Pittsburg Testing Station, consists of a series of five 16 c. p. lamps. These leads are then, after independently traversing the cartridge file at the initial and final points, jointly connected to one of the poles of the primary coil of the induction coil through which the current passes to the return conductor. The secondary coil of the induction coil is then connected by one pole to the two platinum terminals and by the other pole to the base supporting the drum as described. As is well known, in the induction coil any change of tension in the primary coil sets up an induced current in the secondary coil, and this mutual induction between the coils results in the production of a higher-potential difference at the terminals of the secondary one so that sparks of considerable length and intensity may be obtained.
The vibration tachometer, by which the speed of rotation of the drum is measured, is connected to an auxiliary shaft which engages the main shaft of the drum by gears, thus preventing any irregularity in recording the speed due to slipping. This tachometer measures the number of rotations of the drum, but as the circumference of the drum is accurately known, the distance which any point on the periphery travels may easily be calculated. Hence, at the highest speed of to5 revolutions per second, the distance of travel is 52.5 meters. At 50 revolutions it is 25 meters. At 86 revolutions, it is 43 meters per second. With this number of revolutions it is possible with this instrument to measure the 300,000 part of a second of time.
A more recent and simpler method of measuring the rate of detonation is that devised by M. d'Autriche,* which was described at the Congress in London, in 19o9, by Dr. A. M. Comey, as follows:
The method of M. d'Autriche depends upon the use of a special detonating fuse having a uniform velocity of 6000 meters per second. A suitable length of fuse, according to the length of the column of explosive to be tested is taken for the test and the exact middle of the fuse is determined by measure-
* Comptes rend. 143, 641 and 144, 1030.
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FIG. 12.-FLAME TEST APPARATUS.
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FIG. 13.-FLAMES FROM EXPLOSIVES.
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FIG. I4.-X-RAY PHOTO OF FUSE.
ment and marked. A fulminate cap containing a charge of 15 grains (1 gram) is slipped over each end of the fuse and crimped securely. The fuse is then laid upon a piece of 32-pound sheet lead (1 1/2 inches X 15 inches 1/2 inch) (38 X 380 X 13 mm.), so that the center of the fuse is about in the center of the sheet of lead, and the point coinciding with the middle point of the fuse is marked plainly on the sheet lead (M). The fuse passes along the entire length of the sheet of lead, and its ends are bent around so that they nearly meet. The two ends of the fuse covered with the detonating caps are inserted a short distance, at two points, into the column of explosive, the velocity of which is to be tested, and the distance between these points accurately measured. This may be called (A). A fulminate cap with fuse or electric connections is placed in one end of the stick of explosive. When this cap is detonated, the explosive wave proceeds through the detonating fuse in both directions and meets at a point (T) where a sharp line is formed by the effects of the detonation itself, and the lead is often broken through at this point. The distance from (M) to (T) is accurately measured and designated as (b). If the two ends of the fuse are detonated simultaneously (M) and (T) fall together, that is, the detonation proceeds at the same rate through the two halves of the fuse and meets at the middle, but when a certain length of an explosive is placed in the circuit we have on one side one-half the length of the fuse and on the other side one-half the length of the fuse plus a certain measured length of explosive. We have thus, letting,—
X = Velocity of detonation of the explosive tested.
V = Known velocity of the fuse (6000 meters per second).
A = Distance between two ends of fuse, or length of explosive tested.
b = Distance between M and T.
Then,—
X=VA/2b or 6000A/2b.
As to the accuracy of the test, it was found that by using the fuse alone (M) and (T) always coincided to within 1/8 of an inch (3 mm.). It is easily seen that errors in measurement will be diminished by increase in the length of explosive tested, and it can be calculated, with velocities of 4000 to 6000 meters per second, using 15 inches (38 cm.) of powder, that an error of 1/4 of an inch (6 mm.) in measurement of the distance (M) to (T), which is a very large one under the conditions, introduces an error in the determination of the velocity of about 5 per cent.
Comey and his associates have tested this method quite fully at the Eastern Laboratory of the Du Pont Powder Co., and have found that it gives not only a ready and accurate means of determining the velocity of detonation through a column of any desired length of explosive, but that it is also possible by this method to determine the velocity with which a detonation wave travels through the air.
It is obvious that the flame-giving qualities of an explosive plays a most important part in its liability to ignite fire-damp and other combustible mixtures, and that, all other conditions being equal, that explosive which gives the shortest flame for the briefest time is most suitable for use. Hence latterly much attention has been given to the study of the flames from explosives and many devices have been constructed by which to photograph them.
Among these is the one employed at the Pittsburg Testing Station where the flame is photographed on a moving film. The charge of explosive is fired from a "cannon," of the type used in the gallery tests, by means of an electric detonator or igniter, but in this test the " cannon " is mounted vertically in a concrete foundation at a distance of about 18 feet from the lens of the camera. To cut off extraneous light rays, so that the tests may be made at any time, the " cannon" is enclosed in an iron cylinder, 20 feet in height and 43 inches in diameter, which is connected with the dark room by a light-tight iron conduit, as shown in Fig. 12. The cylinder, or stack, is provided with a door in the side, through which the " cannon " can be loaded, and with a vertical slit, 8 feet in length by 2 inches in width, which is so placed that its vertical center coincides with that of the conduit and also with that of the lens by which it is viewed. At the time of firing the top of the stack is covered with black paper. The conduit is closed, at the point where it ends in the wall of the dark room, by a shutter.
The camera consists of a drum on which the sensitized film is mounted, an electric motor by which the drum is revolved at a known rate, a quartz lens, a semi-circular shield in which a stenopaic slit has been cut, and a shutter by which to control the slit in the shield. All of these, except the motor, are enclosed in a light-tight box. The semi-circular shield is placed close to and concentric with the drum to prevent any light reaching the film except that passing through the stenopaic slit. A lens of quartz is used because it focuses not only the visible light rays, but also those invisible violet rays which occur to a large extent in the flames from explosives.
By means of a tachometer both the number of revolutions per minute of the motor and the peripheral speed of the drum are directly read off. The maximum peripheral speed of the drum is 20 meters per second, and this rate is employed when detonating explosives are tested, but with slow-burning explosives the drum is run at a slower rate. At the 20-meter rate 1 millimeter width of flame equals 0.05 milliseconds of time, and as the measurements of the flame photographs are read to the nearest quarter of a millimeter the smallest time interval measured is the 0.0125 milliseconds. The charge of explosive used in the test is 100 grams, and these charges are fired both with and without stemming.
The results of this test on black-blasting powder and on a permissible explosive are shown in Fig. 13. By the term permissible explosive is meant an explosive which has satisfactorily passed all the prescribed tests at the Pittsburg Testing Station, and is regarded as suitable for use in coal mines.
One of the most novel of modern tests is that devised by J. Thomas,* who has employed the X-rays for ascertaining the condition of the powder core in Bickford or running fuse. The cause of misfires and delayed ignitions has been the subject of much speculation, and among other theories proposed was that of a break in the continuity of the powder cores. In Fig. 14, which is a copy of Thomas' X-ray picture, the interruption of continuity in two pieces of the fuse shown is very apparent.
* J. Chem. Met. Soc. S. Africa 9, 183; 1908.