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XLI. Wave Propagation of Magnetism. 

 By John Trowbkidge*. 



"T^ARIOUS attempts have been made to discover a wave- 



* propagation of magnetism along bars of iron or around 

 rings of this metal f. The ordinary method of investigation 

 has been to subject the iron to alternating currents of a certain 

 frequency, and to search for nodes along the extent of the bar 

 or ring. The search has been conducted by employing small 

 secondary coils of wire connected with a ballistic galvanometer 

 or with telephones. 



I have lately examined this question with a new instrument 

 which I have termed a phasemeter, and which seems to me to 

 be of great use in the subject of alternating currents. It 

 consists of an application of Lissajous's method of studying the 

 vibration of tuning-forks to two telephones which take the 

 place of the tuning-forks. On the diaphragm of each tele- 

 phone is affixed a mirror ; and the axes of the telephones are 

 so turned that the excursions of a spot of light, reflected from 

 both the mirrors of the telephones, represent the figure pro- 

 duced by a combination of two motions at right angles to 

 each other. In the telephone which I have employed, the 

 diaphragms are about three inches in diameter, and are 

 clamped delicately around the edges upon their support by 

 little screw-clamps, which can be moved about until the dia- 

 phragm vibrates in unison with the alternating currents which 

 are employed. Professor Eli W. Blake % has described a 

 method of making the vibrations of a telephone-diaphragm 

 visible, And since I began to work upon this subject 

 M. Wien§ has described an instrument which he terms an 

 optical telephone, which he employs for the measurement of 

 alternating currents. In his method a stylus connected with 

 the centre of the diaphragm touches a light mirror which is 

 placed upon a flexible support, and thus the motions of the 

 diaphragm are amplified. Great care must evidently be taken 

 that the support of the mirror should move in unison with the 

 stylus connected w T ith the diaphragm. Professor Blake's 

 method is an extremely sensitive one : for my purposes, how- 

 ever, I have found the method of clamping the diaphragm 

 around its edges at suitable points sufficiently sensitive, and I 



* Communicated by the Author. 



t Dr. Harold Whiting, Proc. American Academy of Arts and Sciences, 

 p. 14 (1881); F. T. Trouton,/ Nature/ November 1891. 

 \ American Journal of Science, vol. xvi. (1878). 

 § Ann. der Fhy&ik und Chemie, No. 12 (1891). 



