221 PROCEEDINGS OP THE AMERICAN ACADEMY 



neto-transrnitter at one end and a magneto-receiver at the other, 

 when a rapidly revolving circuit-breaking wheel is interposed to 

 interrupt the current as many as thirty times per second and 

 upwards. At high rates of circuit-breaking the high-pitched note, 

 due to the interruptions, is simply added to the vocal sounds 

 transmitted. 



In practice, however, a slight modification of the apparatus de- 

 scribed was found to be necessary. As had been surmised when the 

 apparatus was originally devised, clicks and scratches w r ere heard 

 in the receiving telephone when the current was rapidly made and 

 broken by the revolving wheel. These often masked the proper 

 sound of the fork. They seemed to be due to microphonic action 

 at the contact of the spring and brass segment; and although we 

 found that this could often be avoided by careful smoothing of the 

 contact surfaces and proper adjustment of pressure, yet it was 

 difficult to keep the apparatus in good adjustment. For this 

 reason, we substituted for the vulcanite wheel a brass wheel having 

 a single insulating segment of vulcanite. The receiving telephone, 

 whose resistance was 115 ohms, was placed in derivation with the 

 brass wheel, so that during the greater part of the time the receiv- 

 ing telephone was short-circuited and no sensible current passed 

 through it, as the wheel had very slight resistance. But whenever 

 the spring rested on the vulcanite segment the circuit through the 

 wheel was broken; and hence a current passed through the receiv- 

 ing telephone and produced a sound. This arrangement obviated 

 the difficulty just referred to, although care had to be taken to keep 

 the contacts in good condition, as otherwise more or less of a 

 scratching sound was introduced. In order to reduce the resistance 

 of the short circuit through the wheel to a minimum, the current 

 was caused to enter the wheel in the following manner. A disk 

 made of sheet copper was attached to the axle carrying the brass 

 wheel. The edge of the copper disk was amalgamated, and its 

 lower portion dipped into a trough of mercury, thus always secur- 

 ing a good contact. When the current was made to enter the wheel 

 in other ways that were tried, as, for example, through a second 

 spring, it was sometimes found that the resistance of the wheel 

 circuit was not sufficiently low entirely to prevent the production 

 of a sound by the derived current that under these circumstances 

 constantly entered the receiving telephone. With the arrangement 

 described the apparatus worked very satisfactorily, no trace of 

 sound being audible in the receiving telephone except when the 



