[ 173 3 
II. On the Specific Resistance of Mercury. 
By Lord Rayleigh, F.R.S., Professor of Experimental Physics in the University 
v of Cambridge, and Mrs. H. Sidgwiok. 
Received April 24—Read May 4, 1881. 
Our experiments on the determination of the British Association unit of electrical 
resistance in absolute measure are detailed in two memoirs communicated to the 
Society.'" The conclusion to which they led us is that 
earth quadrant 
1 B.A. unit =*9865 --—--, 
second 
but this result differs considerably from that obtained by some other experimenters, 
the original Committee included. Although in the present state of the question it 
is not desirable that the B.A. unit should fall into disuse, there can be no question as 
to the importance of connecting it with the mercury unit introduced now more than 
twenty years ago by Siemens. It will then be possible, as recommended by the Paris 
Conference, to express our absolute measurements in terms of mercury, by stating 
what length of a column of mercury at 0° of 1 square millimetre section has a resist¬ 
ance of 1 ohm. Accordingly the experiments about to be described relate to the 
expression in terms of the B.A. unit of the resistances of known columns of 
mercury at 0°. 
This investigation was the more necessary, as the principal authorities on the 
subject, Dr. Werner Siemens and Dr. Matthiessen, had obtained results differing 
by as much as *8 per cent. 
The earlier determinations of Siemens were vitiated by the assumption of an 
erroneous value (13*557) for the specific gravity of mercury, a constant which it is 
necessary to know in order to infer the mean section of a tube from the weight 
of contained mercury. The error, pointed out by Matthiessen, was afterwards! 
admitted by Siemens, who gives as the corrected expression of the relation between 
the two units, 
1 mercury unit='9536 B.A. unit. 
On the other hand, the independent measurements of the resistance of mercury by 
Matthiessen and HockinJ gave 
* Proceedings, April 12, 1881 ; Pliil. Trans., 1882, Part II. 
t Phil. Mag., xxxi., 1866. 
f Reprint of British Association Reports, p. 114. 
