Take two small carbon rods, p p, if procurable, if not, use two
ordinary nails, and connect them up in the circuit of the battery; lay
them upon a thin box so that the rods or nails cross each other, as in
Fig. 17; insert the electromagnet in the circuit; move the coils out a
little beyond the ends of the cores, lay a thin iron plate over the
ends of the coils, then jar the box upon which the bars, p p, are
laid, or drop a pin upon it, or scratch it with a piece of paper, and
the sound will be heard by placing the ear against the iron plate
resting upon the coils of the magnet.