Page:Popular Science Monthly Volume 14.djvu/146

134 cut-out telephones. The principle upon which they act may be thus briefly stated: In vibrating, the diaphragm cuts from the circuit resistances which are proportional to the amplitude of the vibrations. A transmitter constructed upon this principle is shown in Fig. 16. A lever, L, of metal, vibrating in a vertical plane, rests at one end upon a strip of carbonized silk, C, which is part of the primary circuit of the induction-coil I. In the course of its vibrations the lever cuts from the circuit parts of the silk, the current passing temporarily through the lever.

Another, acting on the same principle, but differing considerably in construction, is shown in Fig. 17. A fine wire, W, of high resistance, is wrapped around a cylinder in a spiral groove.

The wire forms part of the primary circuit of the coil C. A spring, S, of metal, in the form of an ellipse, is fastened at one side to the diaphragm, while the other side presses against the uninsulated wire upon the cylinder. The diaphragm, in moving toward the right, flattens the spring, making it impinge upon a greater number of convolutions than it would if the motion were in the opposite direction. The resistance of the circuit depends, therefore, upon the position of the centre of the diaphragm. The disadvantage of this arrangement is, that either a whole convolution or none at all is suppressed from the circuit, rendering the current rather more intermittent than pulsatory.

In Fig. 18 a similar spring rests upon a narrow strip of metal on the surface of a glass plate. The film is shown in perspective at F, and consists of a fine strip of the silvered surface of a mirror, the rest of the burnished metal having been removed.

The action of this instrument is similar to that of the instrument shown in Fig. 16.

Still another form of short-circuiting telephone is shown in Fig. 19. A spiral spring, W, is wrapped about a cylinder, the diaphragm pressing against the last turn, so that in vibrating the convolutions approach or recede from each other. A very slight motion of the diaphragm is sufficient to cause the first few coils to come together; and in general the number of coils that thus touch each other is dependent upon the amplitude of the diaphragm's motion. The wire is included in the primary circuit of an induction-coil, so that the resistance of the circuit fluctuates as the diaphragm vibrates.

—Telephones in which static charge, instead of current strength, is made to vary in unison with the vocal utterances, have also been tried with success by Mr. Edison. The forms shown in Figs. 20 and 21 differ only in construction, not in principle.

The former consists of a circular vocalizing chamber, with mouthpiece at V. The chamber is surrounded with plates, which are connected with each other and to the ground. These plates are free to vibrate, and are shown in the figure in section, as at P'. Immediately