:i58 



./. K. Roberts: 



llie couples obtained were the following 



Revolutions per 

 second. 



43.96 - 



33.48 - 



21. 9S - 



13.G - 



Deflection of liffht 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* 



Tliese result.s are plotted in Figure 1. 



Using the values at tAvo 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, «/^=l-5x 10*, 

 We liave therefore 



= 2-772 xlO-* E.M.U.) ,.., 

 = 92-5 cm. / ^''> 



Substituting these values in equation (1) we calculate the 

 -couples \lf for difftrcnt values of N, Avith tlie following results : — 



N = 20, ./, = l-87x 10 

 N = 30, ./. = 2-36x 10* 



