If you do not have the time to make A vibrator or electrolytic interrupter for A spark coil, A common electric doorbell makes a good substitute. Connect one of the primary wires to the binding-post of the bell that is not insulated from the frame, and the other primary wire to the adjusting screw on the make-and-break contact of the bell, as shown in the sketch. The connections are made from the batteries to the bell in the usual manner. - Contributed by Ralph Tarshis, Brooklyn, N. Y.