1158 



J. K. Roberts: 



llie couples obtained were the following : — 



Revolutions per 

 second. 



43.96 

 33.48 

 21.98 

 13.G 



Deflection of light on rever- 

 sing direction of rotation 

 of copper. 



- 97.7 cm. - 



- 94.5 cm. - 



- 76.2 cm. - 



- 49.35 cm. - 



Couple 

 (absolute) 



2.69 X 10'' 

 2.26 X 10* 

 1.93 X 10* 

 1.35 X 10* 



These results are plotted in Figure 1. 



Using the values at two points we may determine p and X from 

 equation (1). From the graph when 



N = 42, i/.=:2-6 X 10^ and wlien N=15, xl;=.\o x 10% 

 We liave tlierefore 



Io=i2-772xl0^ E.M.U. 

 X— 92-5 cm. 



} (2) 



Substituting these values in equation (1) we calculate the 

 ••couples \p for different values of N, with the following results : — 



N = 20, i//=l -87x10 

 N=.30, V. = 2-36x 10^ 



