Ox THE ABSOLUTE UNIT OF ELECTRICAL EESISTAXCE 149 



with the coil, and hence makes the deflection greater than it should be, 

 and the absolute value of the Ohm too small by a very small quantity. 

 The currents induced in the suspended parts also act in the same 

 direction. Neither of these can be estimated, but they are evidently 

 very minute. 



The mere fact that this small magnet was attached to a comparatively 

 large mirror which was exposed to air currents could hardly have 

 affected the results, seeing that the disturbances would have been all 

 eliminated except those due to air currents from the revolving coil, and 

 which we are assured did not exist from the fact that no deflection took 

 place when the coil was revolved with the circuit broken. In revolving 

 the coil in opposite directions very different results were obtained, and 

 the explanation of this has caused considerable discussion. As this is 

 of fundamental importance I shall consider it in detail. 



The magnet was suspended by a single fibre seven feet long, and the 

 deflection was diminished by its torsion -00132. No mention is made 

 of the method used for untwisting the fibre, and we see that it would 

 require only 2-11 turns to deflect the needle 1 from the meridian. 

 To estimate the approximate effect of this, we may omit from Maxwell's 

 equation * all the other minor corrections and we have 



GKw cos <f _ GKw ]_ 



: *tan?>(l + /)/7~ $t "\nearly, 



1 ; 



sin 



where we have substituted <p /3 for <p in Maxwell's equation in the 

 term involving t. In this equation <p is measured from the magnetic 

 meridian; but let us take (p as the angle from the point of equilibrium. 

 Then tp' = <p' + a and (p" = <p" , where <p' and (f ' are for negativa 



OJ 



rotation and (p" and <p" for positive rotation and = arc sin 



Let 



Then CR = 



CR" = 



_ 



tan 4'" (1 + ' 

 R,= l(R' + R"}. 



Where R' and R" are the apparent values of the resistance as calculated 

 from the negative and positive rotations, and R, is the mean of the 



Reports on Electrical Standards,' p. 103. 



