Page:Popular Science Monthly Volume 14.djvu/144

132 being regulated by the screw S. The attraction between A and B varies with the distance between them. When, in vibrating, A moves toward B, the attraction rapidly increases, and B lessens its pressure upon C. During a motion in the opposite direction the attraction diminishes, and B, drawn by the screw S, increases its pressure upon C.

A similar contrivance is illustrated in Fig. 8. The diaphragm carries an armature, A, which by its motion changes the potential of two electro-magnets. These changes in magnetism cause a bar situated in their magnetic field to reproduce the original vibrations. The ends of the bar are held by magnetic force against two pieces of carbon, c and c These and the bar are included in the primary circuit of an induction-coil. The resistance of the circuit decreases when the bar is drawn up, and increases as the bar descends.

—The device of using several pieces of the semi-conductor instead of one was early tried by Mr. Edison. He found in general that the loudness of the sound was increased by thus multiplying the number of contact-surfaces, but also that the articulation was impaired. Instruments of this nature have since become known as microphones, though it is not probable that faint sounds were ever augmented through their agency. Fig. 9 shows one of the first forms of the microphone, invented by Mr. Edison, April 1, 1877. Four pieces



of charcoal are used, C C, etc., each supported by an upright spring, as at S and S'. The piece of charcoal nearest the diaphragm impinges upon a disk of carbon, which is fastened to the centre of the diaphragm. The primary wires of an induction-coil are attached to the diaphragm and the spring S'. The circuit is then completed through the semi-conductors. Other forms are shown in Figs. 10 and 11. the former has two carbons, separated by a plate of metal. The latter has three contiguous pieces of carbon.