G. Key by which the current can be shut “off” or “on,” without change of position of the conductors. It can also be used by vibrating it backwards and forwards as an “interrupter.” I, I. Binding screws, to which are attached the conducting wires and sponge-holders, &c.] [SN: The Voltaic Battery.] _The Voltaic Battery_ (see Fig. 6, p. 20) has its cells arranged in the interior of a mahogany case, and in use they are hidden from view and from danger, but I now partially expose them by removal of the tray for holding the sponges and accessories. Their connecting wires are brought across the under surface of the element board, which is made to move upon hinges that, when necessary, the cells may be examined, but at other times this element board is held in position by a bolt, and it should never be needlessly disturbed. These wires conduct the current through the graduating dial, and the position of the needle of this dial determines from how many of the cells the electricity shall be allowed to reach the binding screws, and from them, by way of the conductors, sponge-holders, or electrodes, the body of the patient,[8] or whether it shall be entirely shut off, as is the case when the battery is not in use, and when the needle stands at “0” (Fig. 7). When the needle points to any stud numbered on the dial, the number of cells marked on that stud are brought into action, and the needle is made just wide enough to touch one of the studs before it breaks contact with the preceding one, and thus the current may be increased or decreased in power without shock, and while the electrodes are held applied to the patient; but if it were not so made a series of painful shocks would be communicated whenever the current was increased or decreased. Should the needle, from forgetfulness, be left when out of use in any other position than at “0,” a guard upon the lid of the instrument prevents its being shut, and the operator has his attention called to his inadvertence. A Voltaic alternative, or change of direction of the current, is sometimes required in treatment, and the commutator of the poles enables this to be accomplished without alteration in the position of the conductors. By pushing forwards or backwards the handle which moves a lever working below the element board the current is instantly reversed, and the alternation of the letters “P” and “N” seen through holes cut in the element board indicates at once not only that there has been a change of poles, but which pole is at the moment negative or positive; whereas in all previous instruments, when the poles have been changed, there has either been no letter marking them, or this letter has really been wrong, and one has had to remember this; and under such circumstances, and examining patients in rapid succession, momentary confusion of the poles was very liable to occur, even to a practised operator. A key enables the current to be shut off or on without removal of the conductors.