378 Experiments ivith Alternating Currents. 



current and abscissae the time. If we divide the curve in 

 three parts AB, BO, CD, such that AB 1 =B 1 C l =€ 1 D (ap- 

 proximately), and measure the resistance with the currents 



Fig. 1. 



AB, BC, CD; then denoting the measured resistance by 

 Pi) p2-> j°3> I have obtained experimentally : — 



(a) Without magnetic field, with frequency 4-6: 



4-6 



Pl = p2 = P3' 



(b) AVith intense magnetic field, with the same frequency, 



Pl>P2> p3- 



Taking p 2 as unity, the^following figures are obtained for 

 four spirals: — 



Pi- 



1-0045 



0-9927 



1-0037 



0-9938 



1-0030 



0-9940 



1-0036 



0-9943 



If we take the currents represented by the curves figs. 2 and 3 

 and measure the same pi, p 2 , /03, we obtain as above, 



Pi>p2>ps'> 



but the differences p x —p2 and p 2 —pB a re much smaller than 

 with the current ABCD. 



My method of experimenting for obtaining the data for p^ 

 p 2 , and p 3 was similar to that of Mr. Griffiths. The alter- 

 nating current was supplied by a small magneto-electric 

 machine set in motion by an electromotor. This current was 

 sent through a Wheatstone bridge, in one branch of which 

 was placed a bismuth spiral. The galvanometer branch 

 contained an automatic double intermittent contact (rotated 

 by the magneto-electric machine supplying the alternating 



