The Faradaic, induced, interrupted, or electro-magnetic current, is the third form of electricity employed in medicine. Faraday, as you will recollect, discovered, that if two metallic wires were so fixed as to be parallel and close to each other, but not to touch; and that if then a current of Voltaic electricity were sent along the first wire, another current appeared in the second. This _secondary_ or _induced_ current, as it is called in contradistinction to the current, the _primary_ or _inducing_ current sent along the first wire is only momentary, but it appears again for a moment when the first current ceases, but in a reverse direction. It is most convenient to wind these two wires round two reels, so as to form separate coils, and to place the primary within the secondary coil. Each single turn of the primary then acts not only on the parallel turn of the secondary wire, but on all the turns near it, and the power of such an apparatus is much greater than that which would be obtained by the same lengths of wire running side by side in a straight line. Our two coils being thus arranged, we pass through our primary wire a succession of electrical currents, and in practice this is accomplished by connecting its extremities with a battery supplying a continuous current, which by an ingenious mechanism we frequently break or interrupt. [Illustration: FIG. 10. Faradaic Battery. A. Cells shown by the removal of the compartment. B. For conductors and accessories. D. Screw regulating the pressure of a spring which modifies the vibration of the hammer, E. E. Hammer vibrating between the electro-magnet, F, and the point of a platinized needle regulated by the screw, G. F. Bundle of iron wires rendered an electro-magnet by the passage of the Voltaic current from the cells, A, through the primary coil, K, within which this bundle of wires is inserted. G. Screw regulating position of a platinized needle. H. The graduator, a stem to which is attached the movable secondary coil, L. The front part of the case has been cut away in the engraving, to show the construction of the induction apparatus. I, I. Binding screws for attachment of the conducting wires, &c. K. The primary coil, fixed upon a pedestal. In the figure, the secondary coil, L, is wholly withdrawn from the action of the primary, and its strength of current depending entirely upon the extent to which it covers the primary, it is evident that the height which the graduator, H, stands above the element board will exactly indicate this strength. L. Movable secondary coil. M, M. Binding screws for attachment of the pedal rheotome, N, for slow interruption. These interruptions are made by the pressure of the operator’s foot upon the spring, P, but in practice they are very seldom wanted, and the fittings are only added to the instrument when specially ordered. O. A spring retaining the secondary coil, L, in any desired position.] [SN: The Faradaic Battery.]