A 1-in. hole was bored in the center of a 2-in. square block. Two finishing nails were driven in, as shown in the sketch. These were connected to terminals of an induction coil. After everything was ready the powder was poured in the hole and a board weighted with rocks placed over the block. When the button is pressed or the circuit closed in some other way the discharge occurs. The distance between the nail points–which must be bright and clean–should be just enough to give a good, fat spark. —Contributed by Geo. W. Fry, San Jose, Cal.

— The Boy Mechanic