A true plan is shown at Fig. 21 A. Through the middle of the wheel a small hole is bored to receive the axle of brass which is to be soldered in place. When the wheel is set up, a metal crank and wooden handle should be soldered fast to one end of the axle. A piece of spring-brass wire is fastened to the block, with a staple, and the lower end bent so that the screw in one binding-post will hold it in place. The upper end of the wire is bent in the form of an [L]. From the other binding-post, through the block and up one support, a wire is passed, the end of which comes into contact with the axle. The current, passing in through one binding-post, is carried through this wire to the axle, then to the wheel, and so on out through the spring-wire and remaining binding-post. When in action the circuit is constantly being broken, as the spring-wire jumps from the end of one tooth back to the face of the next tooth. The pulsations are increased or diminished by the fast or slow speed of the wheel, as regulated by the hand motion. The strength of the current is regulated by the force of the battery and should be controlled by a water resistance, as described for the medical battery, or shocking-coil. The interrupter, shown in Fig. 22, is built up on a block six inches square and seven-eighths of an inch thick. A circle is cut from sheet-lead and laid on the face of the block, through which pins, or steel-wire nails, are driven. The lead circle is five inches in diameter and half an inch in width, making the inside diameter four inches. The pins or nails are driven a quarter of an inch apart, and should be properly and accurately separated, so that an even make-and-break will be the result. It is not necessary to bore holes in the lead, but the pins or nails should be driven clear through it, so that perfect contact can be had by the metal parts coming together. Otherwise the apparatus would be useless. Over the circle of pins a brass bridge is erected, so that the cross-strips will clear the heads of the pins. A hole is bored at the middle of the bridge so that the revolving axle will pass through it. The axle is made from a piece of stout wire, or light rod, and near the foot of it, and about half an inch above the base-board, a disk of metal is soldered fast. A piece of spring-brass wire is attached to this disk, so that when the axle is turned the end of the wire trips from pin to pin, thus making and breaking the circuit. The upper part of the axle is bent and provided with a small wooden or porcelain knob.