-310 LORD RAYLETGH AND MRS. H. SIDGWICK OR THE ABSOLUTE 
the value (M 0 ) applicable to the small circuit touched by the inner brush. The area 
of this is \tt (2'096) 3 . For the first and second series we have 
M »=(IvS? (2-0 96 )==-S3G 
For the third series in like manner 
M 0 =-534 
Thus finally for the first and second series 
M —M 0 =214-569, 
and for the third series 
M —M 0 = 110-392 
The resistance coils. 
§ 26. In all three series the resistance h, fig. 3, was a German-silver coil of about 
iV, referred to for brevity as the [yj,]; anti the resistance a was composed of three 
resistances in multiple arc, the first two being standard singles, and the third a 
resistance such as 7 B.A. units taken from a box. To make the necessary change, 
according to the plan already explained in § 9, the 7 would be replaced by 8. The value 
of a is of course determined principally by the unit resistance coils, and only secondarily 
by the resistance taken from the box. 
The third element of the system of resistances was varied in the different series. 
In the first series c was a [10], in the second series it was [l0]-|-[5]-|-[l], and in the 
third series [10]+[5]+[5']. Besides the standard singles, whose values at various 
temperatures was already known in terms of the mean B. A. unit, we had to determine 
accurately the values of the [x\j], the [10], the [5], and the [5'], as well as the small 
resistances of the various connecting pieces employed. 
The [10] has been determined in various ways, but principally by means of the 
device referred to in the former paper.'" Three German-silver wires of about 3 units 
each are wound on the same tube, and their terminals are so arranged that by means 
of a base board containing mercury cups they can be combined either in multiple arc 
or in series. In the former combination they are compared with a standard single, 
and the resistance is found to be (say) 1 -f-a, where a is small. The coils are now 
without loss of time combined in series, a change which can be effected in a moment. 
The resistance in series is very approximately 9 +9a; by the addition of the standard 
single it becomes 10fi-9a, and can now be compared with the [10]. If the difference 
observed be ft we have [10]=10+ 9a+/3. By this method it is easy to obtain an 
accuracy of at least to o o o - 
* Phil. Trans., Part II., 1882, p. 69?. 
