LAWS. — COEFFICIENTS OF SELF-INDUCTION. 261 



XIII. 



CONTRIBUTIONS FROM THE PHYSICAL LABORATORY OF 

 THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY. 



XLII. — ON AN APPARATUS FOR THE MEASUREMENT 

 OF COEFFICIENTS OF SELF-INDUCTION, AND THE 

 INVESTIGATION OF THE PHENOMENA OF ALTER- 

 NATING CURRENTS. 



By Frank A. Laws. 



Presented by Charles R. Cross, January 10, 1894. 



The increasing use of alternating currents in telephony and elec- 

 tric lighting has, during the last few years, directed attention to the 

 measurement of coefficients of self-induction. In this paper certain 

 apparatus devised for this purpose, together with methods for its 

 use, will be described. Further discussion of the various methods 

 of measurement proposed and of experimental results obtained is 

 reserved for a later article. 



For investigations pertaining to telephony it is particularly desira- 

 ble to measure inductances by means of alternating currents of suit- 

 able known frequencies, and of the same order of magnitude as those 

 used in practice. In all such work the reduction of the results is 

 greatly simplified if the E. M. F. employed follows the sinusoidal law. 



Therefore the aim at the outset was to produce a dynamo whose 

 E. M. F. should vary in this manner, and whose frequency of alterna- 

 tion should be about that which is normal in telephonic work. Under 

 these requirements there have been designed in the Rogers Laboratory 

 two dynamos, or arrangements of dynamos. The first of these was 

 planned during the winter 1890-91, and was used by Mr. L. Derr in 

 connection with his thesis in 1891-92. This machine was provided 

 with a stationary armature having Lord Kelvin's zigzag winding on 

 the outer surface of a hollow cylinder. This was placed in the gap 

 between a star-shaped piece of iron and a disk with a corresponding 

 star-shaped opening, the two pieces being similarly and concentrically 

 placed and forming the pole pieces of the machine. Owing to the 



