The following is a description of a rotary interrupter which, when neatly constructed, should give higher frequency results than may be obtained with the ordinary type of vibrator. The first requisite is a small but well made electric motor, of a type similar to those sold by electrical supply dealers for toys. It may be necessary to remount the motor on a larger base. With two or three cells of battery, the motor should develop from one to two thousand revolutions per minute even when some friction is applied to the shaft.

Procure a piece of extra heavy brass tubing, 1" in diameter, and 3/4" long. Smooth both edges, and solder securely to one end a disc of sheet brass of same diameter as the outside diameter of the tube. This will now resemble a cover to a small round metal box. Find the centre of this disc and mark off into four equal parts. With a knife blade, file or jeweller's hack saw, cut four slots in the surface of the cylinder, as if making four segments for a commutator. Then remove two opposite segments of the tube, using great care not to weaken the joints of the two remaining segments. On a piece of whitewood or pine, 3/4" thick, place the end of the brass tube, and sketch thereon the outline of a plug which shall be a tight fit for, and at the same time comform to, the cylindrical contour of the tube. This plug should be secured in place with brass brads or very small brass screws. Measure the diameter of the motor shaft and drill a hole through the plug so it will fit on the shaft like a pulley. Perhaps an easier way to make the plug would be to find a spool of the right size and file slots in it for the brass tube.

There are to be two brushes, one directly opposite the other. These may be of fine copper or brass gauze, or thin spring brass. These are bent to an L shape, the bend at the lower end being attached to the base board with two round head brass screws, one serving for connection with coil.

Connect one terminal of the coil battery to one brush and the other brush to one terminal of the primary winding, and the second terminal of the primary to the other terminal of the battery. A separate set of batteries is used to drive the motor. It will be seen that every revolution of this "commutator" means two interruptions of the current because, when the brushes rest on the brass segments, a circuit is established, and when on the wood the circuit is broken. After experimenting for a time with this interrupter, the amateur may make a cylinder with more segments, which will of course increase the frequency.

In an induction coil, the instantaneous values of the secondary are proportional to the product of the ratio of transformation and the instantaneous pressure upon the primary (disregarding iron losses in the core.) To secure the greatest efficiency, the primary current should be broken as soon as it reaches its maximum value. And when the break occurs, it must be quick and "clean cut" as it were, so as to give the highest voltage in the secondary. It will, therefore, be seen that the above described " commutator interrupter " can be changed and improved somewhat in the design of the segments, by making them wider with less space between so as to give a greater period of time in the " make " and less in the " break " but it would be a good idea to first make one as described, and from it become acquainted with its action.