ministration of galvanic and faradic currents, whenever both of these currents are useful. They may be elicited simultaneously by placing the double switch in a parallel position so as to rest its right portion of the one switch-spring on the flat plate marked with letter G (galvanic) and the left portion of the other switch-spring on flat plate marked F (faradic). The faradic apparatus is started, and all switches pertaining to the galvanic as well as to the faradic apparatus must be set for actual work. The two currents are then to be regulated each by its own controller until the desired degree of strength of each current is reached; the galvanic by the graphite controller and the faradic by the controlling-screw which moves the outer helix of the Du Bois-Reymond coil (see Fig. 69), and by placing the double switch on the button P or s (prim. or second), respectively. For using the simple galvanic current move the double switch to the extreme right, so as to rest one part of the double switch on flat plate G, and the other on the button in the middle. For faradic currents move the double switch to the extreme left so as to rest on the flat plate F, with the button in the middle. The second double switch is the commutator or pole-changer with index N always pointing to the negative pole, whether it be turned to the left or right terminal post (conducting-cord) or whether used in simple or combined currents.-ED.] We rarely, however, have to produce an abrupt reversal of the current; on the contrary, it frequently occurs that we have to avoid it. The double switch and current-controller (Figs. 44 and 45) allows of the current being reversed in a progressive manner. The pointer directed upon the button marked with the highest figure is left in place and the other is turned around the dial; the instant that the latter pointer reaches the button marked with the same number as that occupied by the other the current is reduced to zero; when the pointer is carried still further the current becomes appreciable once more, but has the opposite direction. We may also carry the former pointer backward to the point where the current becomes zero, and then turn the outer pointer forward. Complete medical batteries provided with galvanic and faradic apparatus, current-controllers, commutators, milliampèremeters, etc., may be set in cupboards or in special cases, it makes little difference which, provided that they are protected from evaporation and provided with the necessary accessories (Fig. 47). We shall have occasion to return later to some of these. For other purposes, such as galvano-cautery, the production of luminous effects, or the working of motors, we have recourse to cells of high electromotive force (bichromate or chromic acid cells). [Such a battery as the universal battery, shown in Fig. 48, is capable of performing all this work. This battery consists of two systems of ten zinc and carbon plates. Each is suspended from two hard-rubber platforms (Fig. 48, f and f') placed side by side in the top of the box. A brass spring is connected to each plate by a screw passing through the platform, and presses against a cylindrical commutator, by turning which the plates can be connected for quantity-i.e., all carbons and all zines con nected together, each lead to a binding-post, d e, e' and d' bons of the first system being connected with the zines of t system by the strip of brass connecting the binding-posts or for intensity-i. e., zincs and carbons being connected other alternately. Alongside of the platform f is another which are mounted the binding-posts e and c', which are by flexible cords carrying pins with the plates of the syste a manner that the cord from the post c' leads to the spring the pin being inserted in a hole drilled into the head of connecting the spring with the plate below the platform. post e carries a double cord, the pins of which may b into any of the screw-heads of the systems of plates, and number of pairs from one to twenty may be brought into ac out interrupting the current. The posts b and b' serve to connect the cords leading to the electrodes. Upon the same platforms are also mounted a polarity-changer and a two-point switch for interrupting the current if it is desirable; these appliances being used where an intensity of current is made use of. At the end of the box a galvano-faradic apparatus is inserted, which may be removed and used separately, and away from the battery. In the lower portion of the box are two movable platforms, which may be raised or lowered by a treadle, to which they are connected by cords running over rollers. Upon these platforms are placed two hard-rubber cups, partitioned off into ten compartments each, and in these the exciting liquid, a mixture of bichromate of potash, sulphuric acid, and water, is contained. These cups being immediately beneath the system of plates, suspended from the platforms, when raised by the treadle cause the plates to be immersed in the fluid, and thus a current is established which varies in strength according to the amount of surface of plates exposed to the action of the acid. A ratchet hook at the bottom of the box, by engaging in the bar of the treadle, will keep the cups at any desired height, so that the foot may be taken off; and by disconnecting the cords of either one or the other of the movable platforms, either one or the other of the cups alone may be raised when only a comparatively weak current is desired. Below the faradic apparatus are drawers for containing the instruments, connecting-cords, etc. The size of the box containing the whole apparatus is twenty-eight inches long by twelve inches wide and sixteen inches high, and is, therefore, not too large to be easily moved. When a more simple apparatus is desired for the purpose of using galvanic currents for medicinal purposes, we may employ an apparatus such as is employed in Figs. 49, 50, 51, and 52.--ED.] We may now recall the fact that the practitioner will require one or two cells in order to work his induction apparatus. For this purpose the trouble is in making a choice. The portable forms of apparatus contain the battery for furnishing the current. For the office we make make use of a bottle-cell, of one Grenet cell (medium or large size), or one of the other cells which we have previously described, such as the Gonda Axo cell, Figs. 40 and 41. [The storage battery has become so generally used, chiefly for illuminating and cautery purposes in medicine, that a description of the principle on which it is constructed is in place, although this has |