ASSOCIATION UNIT OF RESISTANCE IN ABSOLUTE MEASURE. 697 
ture. The mean B.A. unit, in terms of which our results are expressed, was defined 
by him, but the differences between the single standards is scarcely of importance 
for our purpose. In calculating the temperature corrections for the two [5]’s, the [10], 
and the [24], which were all of platinum-silver wire, the co-efficient '0003 per degree 
has been used. The temperatures were those of the water in which the coils were 
immersed. They never differed much from the temperature of the room, and were 
referred to a Kew standard. The results of three comparisons, executed by Mrs. 
Sidgwick, are as follows :— 
Resistance in mean B.A. units of platinum-silver standard at 13°. 
July, 1881 . 23'9326 
September, 1881 ..... 23'9341 
November, 1881 . 23‘9348 
In February, 1882, a fourth determination was executed by myself, in which a 
different method was employed. Five coils approximately equal to each other and to 
five units were arranged in a closed case upon a tube of brass. The ten copper 
terminals emerged below from the ebonite bottom of the case, and rested in mercury 
cups upon an ebonite base-board, which was so arranged that by a single motion the 
terminals could be transferred from one set of cups which combined the coils in series 
to another set which combined them in multiple arc.'' 5- In this way resistances are 
obtained in the proportion of 25 : 1, independently of any exact equality between the 
single coils ; for it is obvious that if the resistance in series is given, the resistance in 
multiple arc is a maximum in the case of equality, and therefore varies little, even if 
the equality be not exact. By the aid of this apparatus the [24] was compared with 
a standard unit, without the assistance of other coils. In the first place [24]+[l] was 
compared with the five coils in series, and in the second place the [l] was compared 
with the five coils in multiple arc. The only precaution necessary is to effect the 
second comparison so quickly after the first that the five coils have no time to change 
their temperature. Two determinations by this method on different days gave as the 
resistance of [24] at 13° 
23'9350, 23-9358—mean, 23‘9354. 
It would seem not impossible that the resistance of [24] has gradually increased, but 
the changes are unimportant. We will take as the resistance with which the absolute 
measurement is to be combined, that found in November 23 "9348 ; so that 
23-9348 B.A. units = 23-612 X 10 9 C.G.S.= 23-612 
second 
Hence, as the result of the investigation, we conclude that 
earth quadrant 
1 B.A. umt=*98651 ——-—-- 
second. 
* I believe that Professor Rowland has used a contrivance of this sort. 
4 it 2 
