1881.] Determination of 'the Ohm in Absolute Measure. 115 



Dr. Schuster, who took all the readings of the principal magneto- 

 meter. Mrs. Sidgwick observed the auxiliary magnetometer ; while 

 the regulation of the speed by stroboscopic observation fell to myself. 

 Dr. Schuster also undertook the labour of the reductions and the 

 final comparisons of our arbitrary German silver coil with the standard 

 ohms. 



The observations were very satisfactory, and at constant speeds agreed 

 better than we had expected. The only irregularity that we met 

 with was a slight disturbance of the zero, due to convection currents 

 in the air surrounding the mirror, the effect of which, however, almost 

 entirely disappears in the means. This disturbance could be magnified 

 by bringing a paraffin lamp into the neighbourhood of the mirror. After 

 about half a minute, apparently the time occupied in conduction through 

 the box and in starting the current, the readings began to move off. 

 Complete recovery would occupy twenty or thirty minutes. In future 

 experiments this kind of disturbance will be very much reduced by 

 increasing the moment of the magnet five or six times, and by dimi- 

 nishing the size of the mirror, both of which may be done without 

 objection. 



The comparison of the results at various speeds requires a knowledge 

 of the coefficient of self-induction L. Nothing is said in the Report as 

 to the value of L for the second year's experiments, but the missing 

 information is supplied in Maxwell's paper on the " Electro-magnetic 

 Field,"* together with an indication of the process followed in cal- 

 culating it. The first approximation to the value of L, in which the 

 dimensions of the section are neglected in comparison with the radius 

 of the coil, is 437,440 metres, but this is reduced by corrections to 

 430,165. The value which best satisfies the observations is consider- 

 ably greater, viz., 456,748. A rough experiment with the electric 

 balance gave 410,000 ; but Professor Maxwell remarks that the value 

 calculated from the dimensions of the coil is probably much the more 

 accurate, and was used in the actual reductions. I had supposed at 

 one time that the discrepancy between the results at various speeds 

 and the calculated value of L was due to the omission of the term in 

 tan 4 0, given above, which would have the same general effect as an 

 under- estimate of L ; but, as has been already mentioned, this term 

 was in fact included in the reductions made by Mr. Hockin, in con- 

 junction, moreover, with the value L=437,440. 



A rough preliminary reduction of our observations showed at once 

 that they could not be satisfied by any such value of L as 437,000, 

 but pointed rather to 454,000, and we began to suspect that the 

 influence of self-induction had been seriously under-estimated by the 

 Committee. Preliminary trials by Maxwell's method with the electric 



* "Phil. Trans.," 1865. 



