Electric primers have been issued to our service for a number of years, but their use has been limited, owing to the unreliability of the usual electric circuits on board ship and the want of a simple and ready method of making the necessary connections. Percussion primers still remain the principal means of firing guns on our vessels, with friction primers as an alternate, and electric primers are carried only as auxiliary or for experimental purposes.
The advantages of electricity are so great, however, that it is intended in the near future that it shall be used exclusively for firing heavy guns and torpedoes, although for use in B.L. rifles of the ordinary type, it may be advisable to carry friction primers as an alternate. In firing by friction or by percussion, quite an appreciable interval of time must elapse between the impulse to fire and the ignition of the charge. This delayed action has been a fruitful cause of inaccuracy in target practice, and with a movable gun platform it becomes a matter of great importance. It is due partly to the flexible laniard usually employed and the travel of the parts of the firing mechanism, partly to a personal equation of the gun captain. By the use of the electric current the first of these causes is very much diminished, the second exists with any method of firing. When the gun is to be discharged from some distance, as in turret guns, or when a torpedo is to be discharged from the conning tower, the superiority of an instantaneous method is most apparent.
With electric primers the danger from premature fire is very much lessened, as it is impossible to fire before the breech is entirely closed and locked. Also, fixed ammunition with percussion primers is liable to explode in handling; with electric primers this danger is removed.
One disadvantage, which some will consider a serious one, must be mentioned. It is the difficulty in locating the fault should a miss-fire occur; whether it lies in the primer, the battery, the leading wires or some of the contacts is not apparent at a glance. The remedy lies in having the primers as nearly perfect as possible (and they can be made fully as reliable as others), and by frequent tests to keep the firing circuit always in order. If it is in order when the gun is cast loose, there is as little likelihood of its failure during the firing as of any other portion of the mechanism. It may be advisable to put a tell-tale or sounder on the firing key circuit, and to provide an auxiliary circuit to be used in case of failure of the principal one.
The greatest difficulty heretofore to the introduction of electric firing has been to secure a reliable source of energy. It has not been considered advisable to use the dynamo circuit, in which case, all guns would be dependent upon a common source, the disablement of any portion of which would disable them all, but rather that each mount should carry its own battery and thus be entirely independent.
Fortunately, a primary battery has been found that answers every purpose. After exhaustive experiments at the Torpedo Station, the Exeter Dry Battery was selected as the one best adapted to the work, and it has been adopted by the Bureau of Ordnance. The elements are carbon and zinc with the exciting liquid held in an absorptive substance. There is no leakage nor are gases given off by the chemical action. The cells are put up very compactly in tin cases and sealed. They have been given very thorough tests for endurance under intermittent service, such as would be required on board ship; for efficiency under abnormal conditions, as when sealed and submerged, and when exposed to excessive heat and cold. Under all circumstances they have proved satisfactory.
The standard firing battery consists of three of these cells connected in series, giving an electro-motive force of 4-4 volts, a current of 5.5 amperes short circuited, and having a battery resistance of 0.8 ohms. The battery is put up in a wooden box and then enclosed in a sheet iron case made perfectly water-tight. This is bolted to the gun mount in some convenient place out of the way of the mechanism. One pole of the battery is put in metallic connection with the mount, the other is brought by a conducting wire to the firing attachment. The primer case being in contact with the gun, the return circuit is through the gun and mount. As the whole external portion of the firing circuit has a resistance of about one ohm and the primers as now designed are ignited with about 0.6 amperes of current, it is seen that the energy of the battery is ample.
A new design of electric primer has lately been adopted by the Bureau of Ordnance for use with B.L. rifles (ordinary type) and is now being issued to the service. It differs from the old standard primer in that it has a single leading wire, the stock itself being the other terminal. The stock is of brass, the external form being the same as that of the percussion and friction primers now in service, so that it can be used with the same spring lock by removing the percussion wedge and putting in place the one for friction or electric firing. When in place in the vent the leading wire projects through a slot in the wedge and is fitted with a cone-shaped brass terminal for connection to the firing circuit. If the issue of percussion primers is discontinued, the present spring lock, which has given a great deal of trouble, can be done away with and a much simpler device used.
The interior arrangement of this primer is shown in Fig. 1. The leading wire (of No. 26 B. & S. insulated wire) passes through the base of the stock and is soldered to an insulated copper disc. The bridge is made between two studs, one being in contact with the disc, the other with the stock by a long, thin copper tang, soldered at the mouth. It is a very fine wire of platinum-iridium alloy (90 per cent, plat., 10 per cent, irid.), 3/16" in length and .002" in diameter, or of an equivalent cross sectional area. The electrical resistance of the bridge is about o. 6 ohms, and it is designed to fire with a current of about 0.6 of an ampere. It is primed with a wisp of dry gun cotton, and the rest of the case is then filled with fine meal powder and closed at the mouth with a paper disc and shellacked.
For use with fixed ammunition an electric primer was designed by the Bureau of Ordnance, and after being perfected by the Winchester Arms Company, adopted for use in the service. All late orders for 4", 5" and 6" rapid-fire cases are to be fitted with these primers. It has the same external form as the present percussion primer stock and is held in the base of the cartridge case in the same manner.
The interior arrangement of the primer is shown in Fig. 2. Connection to the firing circuit is made through the firing-pin which, when the breech is closed, is held tightly against the insulated head of the central contact post. The bridge is made between two copper rings, one being in contact with the central post, the other with the primer stock. The outer ring is held firmly in contact with the bridge by a cap-shaped spring, which also serves as a gas check. The dimensions and electrical properties of the bridge are the same as in the primer for B.L. rifles.
These primers are now made with such care and the shop inspections are so exacting as to insure their being almost perfect.
FIRING ATTACHMENTS.
For use with electric primers in R.F. guns with fixed ammunition an electric firing attachment has been adopted to take the place of the ordinary percussion firing mechanism. It consists of a steel case screwing into the breech-plug and carrying through its center an insulated steel pin. Into this case screws a transverse arm also carrying an insulated pin. Steel springs hold the axial pin firmly against the primer head in the base of the cartridge case, and the transverse pin against one terminal of a lug bolted to the breech of the gun.
This lug takes the place of the lug which holds the percussion trigger box in place. Its other terminal, a binding post, is connected to one pole of the battery by an insulated leading wire. It is also fitted with a screw socket to receive the double pole terminal of the firing-key.
As the breech plug is turned into the locked position, the transverse arm revolves and the pin comes in contact with the terminal on the lug; the circuit can then be completed through the firing-key. The gun cannot be fired until the breech is locked, for the circuit can only be completed at that point.
The firing-key consists of a split, wooden handle, hinged at one end, the contact points being held apart by a spiral spring. It is wired with a duplex flexible copper conductor. A brass case encloses the end of the key, so arranged as to take the strain off the connections of the leading wires. A ring is also provided, which, being threaded on the finger of the gun captain, allows him free use of his hand.
The firing-key terminal screws into a water-tight socket on the lug, and remains permanently in place while the gun is cast loose.
As a premature fire might occur as soon as the breech was locked if there should be a short circuit in the firing-key circuit, it has been thought necessary to place a safety socket in the wire leading from the battery to the lug. This consists of an ebonite case with spring socket slipping over a cone-shaped terminal. The wire is secured to the oscillating slide and to the lug by clamps, and is of such length that the circuit will be broken by the recoil of the gun. It is intended that the circuit shall be completed at the safety socket only when the gun is ready to fire.
For use with B. L. rifles (ordinary type), a similar attachment lug is bolted to a convenient place on the mount. One terminal is connected to a pole of the battery, the other by a leading wire is carried to a clamp on the breech face of the gun, and thence to the primer terminal. It has a spring socket in an ebonite case to receive the primer terminal, and the connection is readily made.
It is very necessary that this connection should not be made until the breech is closed and locked to avoid premature fire, but to insure against such accidents even after the breech is locked, it was thought necessary to place a safety socket in the battery wire, similar to the one used with rapid-fire guns.
The firing-key and terminal are the same for all guns, and all sockets, keys, clamps and terminals are interchangeable.
As the firing-key circuit is only in place while the gun is cast loose, a screw cap is furnished to protect the interior of the socket at other times. All wire used for firing circuits should be of the best quality. The insulation should be of rubber or okonite, and all wires should be tinned. Cotton braid should not be allowed between the wire and the insulation. The sizes of conductors, whether solid core or stranded, should not be less than No. 18 B.S. gauge. For the firing-key circuit the insulation should be braided over all, on other circuits it should be taped.
Electric firing attachments have been fitted to all the 5" R.F. guns now in service, and they are being made for the 4". The heavier guns of the Monterey, Minneapolis, Columbia, Cincinnati and Raleigh have also been so fitted.