ELECTRICAL MEASUREMENTS. 289 



h = 2mld 



Multiply numerator and denominator of the second member of 

 this equation by ( d 2 -f- i 2 ) 2 and we have 



d z l* /* 

 <* + li + JL 



1 JJ 2 l6 



h = 2ml d. 



In this expression / 4 /i6 may be dropped, since / is small 

 compared to d, and / 4 is very small compared to d^. There- 

 fore 



Substitute this simplified value of h in equation (ii), and we 

 have 



The large magnet may now be placed nearer to ns (Fig. 211), 

 say at distance d v the corresponding angle of deflection being 

 <$> v and we have 



The uncertain quantity /, which is the distance between the 

 poles of the large magnet, may be eliminated from equation (v) 

 with the help of equation (vi), giving 



ml d 5 tan <f> d, 6 tan <f>, 



. - - * ' 1 I VII I 



If 2(d 2 - d?) 



Observations and calculations. The quantity /, equation (i), 

 is observed and K is calculated from the measured mass and 



dimensions of the large magnet, leaving only ml and H' un- 

 20 



