﻿560 Mr. Carl Barus on Telephonic 



Contact is made by aid of two flat springs A and B, on 

 moving the plate C between them by aid of the handle or 



Fig. 1. 

 ,6 A 



.— ® j m v 



guide D. C being wider on one side than on the other, the 

 circuit aEBCsra is first closed ; after this the circuit 

 aT e AG sra is closed ; and since the shorter side of the 

 key C is provided with a serrated edge, sounds will in general 

 be heard in the telephone, T, on pushing the plate C quite 

 past the springs A and B either from left to right or in the 

 opposite direction. If the resistances r and B, are large as 

 compared with the rest of the circuits, the telephone will 

 cease to respond when e/E = r/(R-\-r), which is the adjust- 

 ment sought. 



As the telephone is less sensitive than a good galvanometer, 

 every precaution must be taken to use it to the best advantage. 

 It is necessary to pass a bifurcated tube from the sound- 

 chamber to the ears, and the key mechanism A C B D should 

 be placed at a distance and operated either by a long 

 stick or cord, or preferably by the pneumatic device so much 

 used in photography. The plate C is best made of copper 

 (like the circuits), and need not be more than 2 centim. long. 

 The teeth should be coarse, say about 0'5 centim. from point 

 to point, and the V-shaped very thin flat spring at the end of 

 A should only just touch the ends of the teeth when C moves 

 across the spring. In this way a minimum of noise is made 

 in the air, the successive taps are sharply individualized, and 

 the circuit is kept closed only during very short intervals of 

 time (avoidance of polarization). Blunt teeth were tried 

 without advantage, and merely to warrant the use of equa- 

 tion (1), § 4. 



The construction of the key * is a matter of great import- 

 ance, for the sensitiveness of the method depends on it. 

 Indeed I have found different telephones behave quite differ- 

 ently relatively to each other when the method of making the 

 contact was varied ; a result agreeing with the experience of 



* Regular rhythmic interruption like that produced by a tuning-fork 

 would possibly increase the sensitiveness ; but the use of such cumber- 

 some apparatus is of course out of the question. 



