ASSOCIATION UNIT OP RESISTANCE IN ABSOLUTE MEASURE. 
667 
should not be liable to escape at high speeds from the effect of centrifugal force. Bits 
of indiarubber tubing were placed round the copper legs, by which a fair fit with the sides 
of the cups was effected; but I thought that it would be an improvement to revert to 
a single contact piece for the mercury cups of no sensible resistance, whose fit could be 
carefully adjusted, and to insert the extra resistances at the connexion of the other 
(outer) ends of the component coils. For this purpose binding screws were employed, 
pressing firmly together the flat copper terminals of the copper wire and of the German- 
silver resistance pieces. It is almost unnecessary to say that these short lengths of 
German-silver wire were doubled upon themselves before being coiled, and that the 
pieces were not touched between a spinning and the associated resistance comparisons. 
Used in this way the screwed up contacts seemed unobjectionable, even though the 
surfaces were not amalgamated. 
On each night and for each speed a set of twelve spinnings was made, six with 
wire circuit open, and six with wire circuit closed. It was usual to take, first, two of 
the former (one with positive and one with negative rotation); secondly, to compare 
the resistances of the revolving circuit and the standard; thirdly, after inserting the 
contact piece and adjusting the indiarubber strap by which it was held down, to make 
the six closed contact spinnings; fourthly, to compare the resistances again; and 
lastly, to complete the open contact readings. Each spinning, it will be understood, 
involved the reading of several elongations (about six for the open contact and ten for 
the closed), from which the position of equilibrium was deduced. 
Table II. (p. 691) gives all the results of the second series, except one for 35 teeth 
on August 27th, which was rejected on the ground that it exhibited such large internal 
discrepancies, as to force us to the conclusion that the contact piece had been inserted 
improperly. It will be seen that the agreement is good except on August 29th, in 
which case the deflections are as much as four or five tenths of a millimetre too small. 
These discrepancies, though not very important in themselves, gave me a good deal of 
anxiety, as they were much too large to be attributed to mere errors of reading, and 
seemed to indicate a source of disturbance against which we were not on our guard. 
The least unlikely explanations seemed to be (l) a change in the distance of the 
mirror from the scale, which unfortunately had not been remeasured at the close of the 
spinnings, though this would require to reach 3 millims. ; (2) imperfect action of the 
contact piece from displacement of mercury or otherwise ; (3) a change of level in the 
axis of rotation. The anomalous result of August 27th seemed to favour (2), while 
on behalf of (1) it must be said that the stand of the telescope and scale as well as the 
support for the suspended parts of the principal magnetometer were of wood. It was 
just conceivable that under the influence of heat or moisture some bending might have 
occurred. 
On my return to Cambridge in October we proceeded to investigate these questions 
with the closest attention. As repeated direct measurements of the distance of the 
mirror and scale were inconvenient, measuring rods (like beam compasses) were provided 
