The introduction into a modern man-of-war of elaborate electrical plant has brought into prominence the necessity of having some kind of apparatus on board ship for testing the condition and efficiency of the plant from time to time. When it is remembered that a "plant" now-a-days includes circuits for search lights, incandescent lamps, for guns, torpedoes, gongs, bells, etc., the necessity for having some instrument wherewith to test these various circuits, or to localize faults in them, becomes apparent.
The leading of the wires on board a vessel is, of itself, a laborious undertaking, for so many precautions have to be observed. Thus, wires must be out of the way of an enemy's shot and shell as much as possible, they must be accessible, must be protected from chafe, salt water or undue strains, and the various circuits must be conveniently located for those who are to use them. As a rule, wires are led below the water line wherever possible, double circuits being used above the water line, and whatever apparatus is put on board ship should also be below the water line if possible.
The most ready means of testing circuits, determining whether or not they are in good condition, whether there are breaks, short circuits, etc., is to measure the resistance of the circuits, or test their continuity by means of a galvanometer and battery. To enable officers in charge of electrical plant aboard ship to make these tests, there has been devised at the Torpedo Station an apparatus which seems to fulfill the requirements of compactness, simplicity and completeness, and which is called the "Electrical Testing-Table." It consists of a stout wooden locker containing the voltaic batteries sent to the ship, to the top of which is secured a rectangular table with all the testing apparatus permanently secured to it. To protect the table from dampness, careless handling, etc., it has a cover fitting over it provided with a lock and key. On the table are secured:
1. A plug switchboard having binding screws for the attachment of permanent wires to gun and torpedo circuits, a return wire, an earth circuit, and the wires from the batteries in the locker. The switchboard is also permanently connected to wires leading to the measuring apparatus (on another part of the table), and by the insertion of plugs at the proper points in the switchboard any circuit may be connected with the measuring apparatus.
2. A phig commutator for connecting the poles of the measuring battery to the measuring apparatus, which is called a rheometer.
3. An arrangement for testing the condition of the batteries at any time.
4. The measuring apparatus or rheometer, with a galvanometer for use with it.
The battery-testing arrangement consists of a resistance coil, an ordinary service fuse bridge, and a contact key, let into slots cut in the top of the table, connected together by wires, in series, and also to the plug switchboard. By manipulating the plugs of the latter, any battery in the locker below can be thrown into circuit with the testing arrangement, and on pressing the contact key (and thus closing the circuit) the fuse bridge should be heated to a bright red color if the battery is in good condition. The fuse bridge is protected from being broken by a piece of glass fitting over the top of the slot.
The rheometer (a plan of which is shown in Fig. 3) is constructed on the principle of the Wheatstone bridge, which is as follows:
Suppose we have a network of conductors, ABCD, arranged as in Fig. 1, the arms AB, AD, DC having known adjustable resistances a, b and r in them, and the arm EC 2i resistance X to be determined. If the resistances a, b and r are so adjusted that there is no deflection shown on the galvanometer G, the points B and D must be at the same potential or pressure ; otherwise there would be a current in the galvanometer circuit, and hence a deflection on the galvanometer.
These values of v can be marked on a scale the ends of which are opposite the ends of the wire BD (Fig. 2), and then when a balance is obtained it is only necessary to observe the reading of the scale 5 as indicated by the pointer P, multiply it by r, the known resistance, and we have the value of the unknown resistance at once. The rheometer is constructed on the foregoing principles, and Fig. 3 gives a plan of the rheometer and its connections, the latter being led under the table wherever possible, and let into grooves on the under side of the table.
QSTV the base board, of hard wood, secured to the testing table. G1G2 is a uniform platinum-silver wire stretched between the binding posts d and G1 and soldered securely to them. NN is a brass bar having the pointer P running upon it, the pointer pressing on the wire G1G2 and making metallic contact with it. In the box W are four resistance coils of 1, 10, 100, 1000 ohms resistance respectively, the ends of the coils being connected to adjacent pieces of the switchplug B, as shown. By this arrangement any one, or more than one, of the coils can be thrown into the circuit of the adjustable arm when desired. RM is the scale, graduated as before described, the extremities of the graduation being abreast the ends of the wire G1G2. W is a metal switch pivoted at the binding post b, and which can be switched on to either of the brass stops marked Tel. and Gal. I is an induction coil ; one end of the primary coil is connected to the stop marked Tel., and the other end, through the interrupter J, to the binding post d.
This interrupter (see Fig. 4) consists of a brass spring a with a soft iron armature at its extremity, the spring being permanently attached to the binding post d. One wire from the battery goes to this binding post. The spring presses ordinarily against the screw c, which works in the frame J. When a current passes through the primary coil, the coil, acting as an electro-magnet, attracts the soft iron armature of the spring and draws the latter out of contact with c. This breaks the circuit ; the coil no longer acts on the iron armature, and the spring flies back against c, thus establishing the circuit again. This making and breaking of the circuit results in a rapid,, vibratory motion of the spring a, the amplitude of the vibration being regulated by working the screw c in or out. This vibratory motion is utilized when using the telephone, as described later on.
One end of the secondary coil of the induction coil I is connected to the piece C of the switchplug B (Fig. 3), and the other end, to the wire between the binding post m and the rod NN. A is a switchplug in the circuit of the astatic galvanometer G. The latter has two coils, one of 10 and one of 100 ohms resistance, either of which can be used ; the galvanometer needle is kept parallel with the plane of the coils ordinarily by means of a small permanent magnet p, placed vertically opposite the zero of the galvanometer scale. A is a plug commutator connected to the measuring battery and to the rheometer by permanent wires. All the connections of the rheometer are shown by dotted lines in Fig. 3. The commutator is used when it is desired to reverse the direction of the current through the rheometer.
Ordinarily, the apparatus would be used as follows : The pins in the plug switchboard having been arranged so as to connect the proper circuit with the wires x1, x2 (Fig. 3), see the pin out at C in the switchplug B, and put the pin in at K, also throwing the switch W on to the stop marked Gal. ; unplug one of the resistances in W (the one that corresponds nearest to the known approximate resistance of the circuit to be measured) and slide the pointer P along the wire G1G2 until the galvanometer shows no deflection. Then the scale reading shown by the pointer, multiplied by the unplugged resistance W, gives the resistance of the circuit or wire measured, plus the resistance of the "leads"; the resistance of the latter, being measured before or afterwards, can be deducted from the total resistance. In plugging or unplugging, the pins should never be pushed or pulled sideways ; a slight turn of the pin will always seat it well in place, or start it from its seat. If this precaution is not observed, the pinholes become warped or enlarged, and the pins will make bad contact.
Although the needle of the galvanometer is held steady ordinarily by its permanent magnet, there may be occasions when, owing to the motion of the vessel or other causes, the galvanometer cannot be depended upon. To provide for such a contingency, a small telephone, with wires attached, is furnished with the testing table, to be used in connection with the induction coil I. To use it, the following changes in the connections must be made: Take out the pin at A' and put it in at C; change the battery wire g from the binding post m to the binding post d, and throw the switch W on to stop marked Tel. ; unplug one of the resistances IV as before. Now connect the ends of the telephone wires to the binding posts G1 and G2 and put the telephone to the ear. The vibrations of the interrupter spring, when they are properly regulated by the adjusting screw, make themselves known in the telephone by a peculiar humming or buzzing sound. Move the slider F along the wire G1 G2 until the humming ceases, when the balance is obtained. The unplugged resistance, multiplied by the scale reading, as before, will give the approximate resistance of the circuit or wire measured. It will usually be found that there are two positions of the pointer P in which the humming ceases, depending upon whether the pointer is moved up or down the wire. The mean of the two scale readings so obtained may be taken as the true reading. For use with the rheometer a battery of four sawdust or other similar cells will be supplied. The current from this battery, however, will not operate the interrupter of the induction coil, and hence when the telephone is used, the firing battery, in the same locker as the other, can be employed. When so used, the circuit should be kept closed as short a time as possible, as the firing battery will soon run down if left on a closed circuit for any length of time. The firing battery should never be used with the galvanometer, as the current is strong enough to fire a fuse, if one should happen to be in the circuit that is being measured or tested; hence it is well to get used to the testing battery whenever the galvanometer is used. With the telephone, only the secondary current goes through the circuit to be measured, and as this is so small, there is no danger of a fuse being fired; hence the firing battery can be safely used to furnish the primary current in this case.
While the rheometer is not so accurate or sensitive as the ordinary form of Wheatstone bridge unless the measured resistance approximates quite closely to one of the known resistances, 1, 10, 100, 1000 ohms, it is better adapted for use aboard ship for the following reasons:
1. There are many fewer resistance coils, whose connections are liable to be broken or the coils themselves injured by burning, short-circuiting, or otherwise ; hence it is more compact and cheaper.
2. There are not so many pinholes to get out of shape by careless plugging or unplugging.
3. It can stand much rougher handling, and hence is not so liable to be injured by the motion of the vessel, shocks due to firing of guns, etc. This last condition, however, must not convey the idea that the rheometer should be handled or used with any less care then a bridge should be. Any instrument of the kind should be handled intelligently and carefully.
Besides the testing table described above, vessels are now supplied with a small " testing dynamo," weighing only four or five pounds, and very handy, having a bell or gong in its circuit. This bell will ring with a resistance of 5000 ohms in the circuit. In the torpedo supply box will also be found a " pocket galvanometer," about three inches in diameter only, and which is a combination of a small battery and a galvanometer. It can be used either as a battery and galvanometer combined or as a galvanometer alone. The testing dynamo and the pocket galvanometer are very useful for testing the continuity of a circuit quickly, testing a fuse or detonator, localizing a break in a circuit, and for many similar purposes. Besides the various instruments already described, officers in charge of the electrical outfit aboard ship will find many articles therein which will serve as material for improvised instruments, either to aid the officer in his routine work or in any special work required, and which will readily suggest themselves to his mind when needed.