2.—Terminals or connections between the cells corroded, so that instead of metallic contact there are oxides of almost insulating resistance intervening in the circuit.
3.—Cells empty, or nearly so.
4.—Filaments of deposited metals stretching from electrode (pole) to electrode (pole).
Also intermittent currents are sometimes produced by loose wires or a broken electrode, which alternately makes and breaks contact when shaken. Inconstant currents are also sometimes produced when batteries are shaken. The motion shakes the gases off the electrodes, thus increasing temporarily the electro-motive force of the battery.
Firing Keys and Shutter Apparatus.—The following is a description of the various firing keys and shutter signalling apparatus, which is used in connection with a system of electrical submarine mines. By means of the former the firing or other batteries may be thrown into circuit at will, whilst by means of the latter the firing battery is thrown in circuit without the aid of an operator, and a signal at the same instant given, indicating that a certain mine of the system has been struck.
Description of a Series of Firing Keys.—At [Fig. 78] is shown a plan and section of a series of firing keys as arranged for firing several mines by observation.
It consists of a strong wooden frame a, of a convenient form for the purpose of attaching it to the firing table by screws through the holes b, b. On this frame a series of keys c, c, c are fixed at convenient intervals. These consist of a strong brass spring firmly screwed to a series of brass plates d, d, d on the front of the wooden box a. From these latter short copper wires pass through the woodwork, and of such a length that, when required, the mine wires may be easily attached by means of binding screws, as shown at f. The inner end of each key is fitted with an ebonite knob (which is shown at c in the section) to insulate the hand of the operator when using the key. On the frame, and directly under each of the ebonite knobs, are arranged a series of metallic points g, g, g, so placed that on either of the keys c being pressed down, a perfect contact is made between it and its respective metallic point; h, h, h are copper wires leading from the metallic points g, g, g through the box, and of such a length that binding screws f, f, f can be easily attached to them when necessary.
A single firing key of an improved form is shown at [Fig. 79]. It consists of a strong wooden box a a, weighted at the bottom with lead in order to steady the key on the table, &c., on which it may be placed; on the inside of the bottom of the box is fixed a piece of ebonite, by which means the metallic point b, and the terminal of the firing key c, are insulated from each other; d d' are two terminals at the end of the box, to which the circuit wires are attached, one of these terminals is connected in metallic circuit to the firing key at c, the other one to the metallic point b; a wooden cover h, fitted with a catch k, protects the connections of the wires; by means of a plate, and catch e e, the key can be rendered inactive, thus preventing the danger of a premature closing of the electric circuit; by means of a spring s a break is always established between the key and the metallic point. It is immaterial to which of the two terminals d d' either wire is connected.
The Morse Firing Key.—This form of key is so well known in connection with the Morse telegraph, that it is not necessary to describe it.
It is usually employed in torpedo work in connection with a testing and firing table.