3. Experiments with the Telephone

Extract

In the telephones used in these experiments the permanent magnets were of the ordinary horse-shoe form, about 4 inches long. No cores of soft iron were attached to the poles, the insulated wire, No. 26, being wound directly round both, in such a way that a current circulating through it followed the direction of Ampère's currents. The vibrating disc was the bottom of a shallow can of thin tinned iron, 2½ inches in diameter, supported directly above the poles of the magnet, and almost touching them.