On the Measurement of Alternate Currents. 343 



the superposition of two such diffraction - patterns as are 

 represented by (35) might be realized, but in general the 

 star-image will be so broadened and disturbed in position by 

 continual atmospheric disturbance (to say nothing of chro- 

 matic aberration in the case of the image being formed with 

 a lens), that the effect on the photographic plate will in the 

 long run be practically the same as would be produced by a 

 uniformly illuminated slit. 



Yerkes Observatory, 



University of Chicago, 



February 1897. 



XL VI. On the Measurement of Alternate Currents by means 

 of an obliquely situated Galvanometer Needle, with a Method 

 of determining the Angle of Lag. By Lord Raylbigh, 

 FM.S* 



IT is many yearsf since, as the result of some experiments 

 upon induction, I proposed a soft iron needle for use with 

 alternate currents in place of the permanently magnetized 

 steel needle ordinarily employed in the galvanometer for the 

 measurement of steady currents. An instrument of this kind 

 designed for telephonic currents has since been constructed 

 by Giltay ; but, so far as I am aware, no application has been 

 made of it to measurements upon a large scale, although the 

 principle of alternately reversed magnetism is the foundation 

 of several successful commercial instruments. 



The theory of the behaviour of an elongated needle is 

 sufficiently simple, so long as it can be assumed that the 

 magnetism is made up of two parts, one of which is constant 

 and the other proportional to the magnetizing force. If 

 internal induced currents can be neglected, this assumption 

 maybe regarded as legitimate so long as the forces are small J. 

 In the ordinary case of alternate currents, where upon the 

 whole there is no transfer of electricity in either direction, 

 the constant part of the magnetism has no effect ; while the 

 variable part gives rise to a deflecting couple proportional on 

 the one hand to the mean value of the square of the mag- 

 netizing force or current, and upon the other to the sine of 

 twice the angle between the direction of the force and the 



* Communicated by the Author. 



t Brit. Assoc. Report, 1868; Phil. Mag. vol. iii. p. 43 (1877). 



X Phil. Mag. vol. xxiii. p. 225 (1887). 



