THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



DECEMBER 1900. 



LIII. An Electric Micrometer. By Philip E. Shaw, 

 B.A., B.Sc, Lecturer in University College, Nottingham*. 



Preliminary. 



IN the summer of 1897, 1 was engaged in investigating some 

 of the conditions which conduce to high efficiency and 

 stability in a telephone system. 



The ultimate product of the system is, of course, vibration 

 of the receiver-diaphragm ; so that, ceteris paribus, any change 

 in the apparatus, or connexions, which produces increased 

 amplitude of that diaphragm, makes for efficiency. 



Suppose now, for instance, that the winding of the receiver- 

 electromagnet is altered, then, passing in constant sound to 

 the transmitter, if we measure the movement of the receiver- 

 diaphragm before and after any such change, we shall be 

 able to judge of the efficacy of the change. 



Such was the idea that led to the making of the measuring 

 arrangement to be described, which has proved to be suitable 

 both in delicacy and adaptability for diaphragm movements. 



I. Experimental Methods. 

 1. Apparatus. 



First Form. — The principle employed is to measure the exact 

 place of contact of two surfaces which, on meeting, complete a 

 circuit containing a telephone. A special spherometer was 

 made with great care; the disk was 8 cm.in diameter, graduated 



* Communicated by the Physical Society : read March 23, 1900. 

 Phil. Mag. S. 5. Yol. 50. No. 307. Dec. 1900. 2 Q 



