ASSOCIATION UNIT OP RESISTANCE IN ABSOLUTE MEASURE. 671 
latter result is on the supposition that the B.A. unit is really 10° C.G.S. If we inquire 
what value of the B.A. unit will reconcile the two results, we find— 
1 B.A. unit='9873 ohm, 
in very close agreement with the measurement described in the present paper. It 
should be remarked that in the comparison of the two thermal results some of the 
principal causes of error are eliminated; and it is not improbable that an experiment 
in which heat should be simultaneously developed in one calorimeter by friction, and in 
a second similar calorimeter by electric currents, would lead to a very accurate determi¬ 
nation of resistance, more especially if care were taken so to adjust matters that the 
rise of temperature in the two vessels was nearly the same, and a watch were kept 
upon the resistance of the wire while the development of heat was in progress. 
[June, 1882.' — Since this paper was sent to the Society, Mr. Glazebrook has worked 
out the results of a determination of the B.A. unit in absolute measure by a method 
not essentially different from that adopted by Howland. The final number is practi¬ 
cally identical with that of the present paper ; and the agreement tends to show that 
the difference between ourselves and Howland is not to be attributed to the use of a 
ballistic galvanometer. 
Heference should have been made to the results of Lorentz.* He finds as the 
value of the mercury unit defined by Siemens 
1 
mercury unit=‘9337 
earth quadrant 
second 
The corresponding number calculated from the results of the present paper with 
use of the value of the specific resistance of mercury lately found (Proc. Hoy. Soc., 
May 4, 1882) is *9413. If we invert the calculation, we find that according to 
Lorentz the value of the B.A. unit would be *9786 absolute measure. The method 
of Lorentz is ingenious, and apparently capable with good apparatus of giving a 
result to much within 1 per cent. Mrs. Sidgwick and myself are at present making 
a trial of it.] 
It will be desirable here to consider briefly some of the criticisms of Kohlrausch 
and Howland upon the method of the original British Association Committee, which 
has been adopted in the present investigation without fundamental alteration. The 
difficulty, remarked upon by Kohlrausch, of obtaining a rapid and uniform rotation, 
has not been found serious, and I believe that no appreciable error can be due either 
to irregularity of rotation or to faulty determination of its rapidity. It has also 
been brought as an objection to the method that a correction is necessary on account 
of the magnetic influence of the suspended magnet upon the revolving circuit. The 
theory of this action is, however, perfectly simple, and the application of the correction 
requires only a knowledge of the ratio of the magnetic moment to the earth’s 
MDCCCLXXXII. 
* Pogg. Ann., 1873. 
4 R 
