672 
LORD RAYLEIGH OH THE VALUE OF THE BRITISH 
horizontal force. If the magnetic moment is very small, the correction is unimpor¬ 
tant ; if larger, it can on that very account be determined with the greater ease and 
accuracy. It is probable that in the original experiments too feeble a magnetic 
moment was used, and that in consequence the suspended parts were too easily 
disturbed by non-magnetic causes; but this might have been remedied without 
increasing objectionably the correction in question. At any rate the larger coil 
of the new apparatus allows the use of any reasonable magnetic moment. 
Perhaps the least advantageous feature in the method is the necessity for creating 
a violent aerial disturbance in the immediate neighbourhood of a delicately suspended 
magnet and mirror. If, however, any deflection occurs in this way, very little error 
can remain when the open contact effect is subtracted from the closed contact effect. 
The difficulty of avoiding a sensible deflection, due to currents in the ring, when 
the wire circuit is open, is connected with a special advantage— i.e., the possibility of 
assuring ourselves that there is no leakage from turn to turn of the coil. In the 
method followed by Howland, for instance, such a leakage would lead to error, and 
could not be submitted to any direct test. 
The correction for self-induction cannot be made very small without a disad¬ 
vantageous reduction of the whole angular deflection ; but so far as the wire is 
concerned it can be calculated d priori, or determined by independent experiment, 
with the necessary accuracy. There is reason, however, to think that the best method 
of treatment is to determine this correction from the spinnings themselves, combining 
the results of widely different speeds so as to obtain what would have been observed 
at a small speed. At small speeds it is certain that all effects of self-induction and of 
mutual induction between the wire circuit and other circuits in the ring will disappear. 
Measurements of coil. 
The mean radius of the coil, being the fundamental linear measurement of the 
investigation, must be found with full accuracy. There has been some difference of 
opinion as to the best method of effecting this. The greatest accuracy is probably 
attained by the use of the cathetometer. The measurement of the circumference of 
every layer by a steel tape has the advantage that the subject of measurement is three 
times as large, and is much less troublesome. The disadvantage is that if a layer be 
not quite even, there is danger of measuring the maximum rather than the mean 
outside circumference. In the present investigation the coil was so large that the tape 
could be employed without fear.* 
Each of the component coils marked A and B had 18x16 = 288 windings, but in 
* The original Committee also employed the tape method. Their measurement of the length of the 
wire when unwound was not in order to find the mean radius, as Siemens and Kohlrausch suppose, hut 
to verify the number of turns. 
