Page:Popular Science Monthly Volume 69.djvu/434

430 electrically. There were two membrane-diaphragm magneto-telephones, which could be used either as transmitters or as receivers. The only difference was that in one a single-pole electromagnet was used and a double-pole electromagnet in the other. The armature of each electromagnet consisted of a small piece of steel spring glued to the center of the membrane which was three inches in diameter. The castings were of brass, mounted on black-walnut base-boards, and the cones were of japanned tin (Fig. 1). A third form of speaking telephone

was shown, intended to be used only as a receiving instrument. The electromagnet was enclosed in a hollow box of iron, and a lid of iron was used as an armature. This lid formed a thin circular metallic diaphragm, resting by its edge upon the rim of the iron box, its central portion not quite touching the pole of the electromagnet underneath. This receiver could be placed in circuit with either one of the