114 Lord Rayleigh and Dr. A. Schuster. [May 5, 



the measurements, inasmuch as no disturbance, due to the rotation of 

 the coil with circuit open, could be detected until higher speeds were 

 approached than it was at all necessary to use. 



One of the first points submitted to examination was the influence 

 of currents induced in the frame. Without altering the speed or 

 making any other change, readings were taken alternately with the 

 contact-pieces in and out. Observations made on several days agreed 

 in showing a small effect, due to the currents in the frame, in the 

 direction of a diminished deflection. The whole deflection being 516 

 divisions of the scale, the mean diminution on making the top contacts 

 was "86 division. When the coil was at rest no difference in the zero 

 could be detected on moving the contact-pieces. 



In these preliminary experiments very consistent results were ob- 

 tained at constant speeds, whether the rotation was in one direction or 

 the other ; but when deflections at various speeds were compared, we 

 were startled to find the larger deflections falling very considerably 

 short of proportionality to the speeds. There are only two corrections 

 which tend to disturb this proportionality — (1) the correction for 

 scale-reading, (2) the correction for self-induction. The effect of the 

 first is to make the readings too high, and of the second to make the 

 readings too low at the greater speeds. According to the figures given 

 by the Committee (Report, p. 106), the aggregate effect is to increase 

 the readings, on account of the preponderance of (1) over (2), whereas 

 our results were consistently of the opposite character. Everything 

 that could be thought of as a possible explanation was examined theo- 

 retically and experimentally, but without success. The coil was dis- 

 mounted and the wire unwound, in order to see whether there was any 

 false contact which might be supposed to vary with the speed and so 

 account for the discrepancy. After much vexation and delay, it was 

 discovered that the error was in the statement in the Report, the effect 

 of self-induction being given at nearly ten times less than its real 

 value. The correction for scale-reading, instead of preponderating 

 over the correction for self -ind action, is in reality quite a small part of 

 the whole. 



At this stage, as time was running short, we determined to proceed 

 at once to a complete series of readings at sufficiently varied speeds, 

 postponing the measurement of the coil to the end. The wire had 

 been rewound without extreme care to secure the utmost attainable 

 evenness, and the condition of the groove was such that a thoroughly 

 satisfactory coil could not have been obtained, even with extreme care. 

 It appeared, however, on examination that irregularities of this sort 

 were nGt likely to affect the final result more than one or two parts in 

 a thousand, if so much ; and many points of interest could be decided 

 altogether independently of this measurement. 



The details of the experiments and reductions are given below by 



