134 THE POPULAR SCIENCE MONTHLY. 



cut-out telephones. The principle upon which they act may be thus 

 briefly stated : In vibrating, the diaphragm cuts from the circuit resist- 

 ances which are proportional to the amplitude of the vibrations. A 

 transmitter constructed upon this principle is shown in Fig. 16. A 

 lever, i, 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 L 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, flat- 

 tens the spring, making it impinge upon a greater number of convolu- 

 tions 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 surfa^'ce 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. • -n^- m 



Still another form of short-circuiting telephone is shown in l^ig. 13. 

 A spiral spring, W, is wrapped about a cylinder, the diaphragm press- 

 ino- 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 m 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. 



CojfDENSER-TELEPHONES.— Telephones in which static charge, in- 

 stead 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 m prm- 



ciple. . • 1, +1 



The former consists of a circular vocalizing chamber, with mouth- 

 piece at V. The chamber is surrounded with plates, which are con- 

 nected 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 



