668 LORD RAYLEIGH ON THE VALUE OF THE BRITISH 
to check the relative positions of the telescope stand and of the upper end of the sus¬ 
pending fibre with regard to fixed points on the walls of the room. But no changes 
comparable with 3 millims. were detected, even under much greater provocation than 
could have existed during the August spinnings. The next step was to examine the 
action of the contact piece. For this purpose the coil was balanced against the standard 
as usual, except that the contact piece was inserted and connexion with the bridge 
made at the other ends of the double coil. It was presently found that the resistance 
did depend upon the manner in which the contact piece was pressed, and that to an 
extent sufficient to account for the August discrepancies. Eventually it was discovered 
that one of the legs of the contact piece, which by a mistake had been merely rivetted 
and not soldered in, was shaky. 
After this there could be no reasonable doubt that the faulty contact piece was the 
cause of our troubles. In all probability the leg became loose on the 27th, in which 
case the earlier results would be correct. Moreover, the final means are not very 
different, whether the spinnings of August 29th are retained or not. This being the 
case, we might perhaps have been content to let the matter rest here ; but in view of 
the importance of the determination, and the desirability as far as possible of convinc¬ 
ing others as well as ourselves, we thought that it would be more satisfactory to make 
a third and completely independent series of spinnings. 
In this series the faulty composite contact piece was replaced by a horse shoe of 
continuous copper, and a check was instituted upon the distance between mirror and 
scale. The opportunity was also taken to make a minor improvement in connexion 
with the auxiliary magnetometer. The somewhat unsteady table on which the telescope 
and scale had stood was replaced by one of stone, and the arrangements for illumination 
were improved by throwing an image of a gas flame on the part of the scale under 
observation. The same number of readings were made as in the second series, but we 
found it more expeditious to take the six open contact spinnings together. At the 
beginning of the evening it was desirable to commence with these open contact 
spinnings in order to give more time for the coil to acquire the temperature of the room, 
which always rose somewhat, although the lamps and gas were lit a couple of hours 
beforehand. Later in the evening we sometimes took the closed contact readings for 
two speeds consecutively, in order that the intermediate resistance comparison might 
serve for both. In other respects the arrangements were unaltered. 
Full details of the observations and reductions are given below. It will be sufficient 
here to mention that the maximum discrepancy between any two deflections at the 
same speed amounts only to of a millimetre, so that the agreement on 
different nights is more perfect than could have reasonably been expected. At the 
lowest speed the above-mentioned discrepancy is less than one part in 3000, and at the 
highest speed less than one part in 6000. No spinnings in the third series were 
rejected, except on one or two occasions when it appeared at the time of observation , 
from the behaviour of the auxiliary magnetometer, that there was too much earth 
