174 
LORD RAYLEIGH AND MRS. SIDGWICK 
] mercury unit='96L9 B.A. unit, 
the mercury unit being defined as the resistance at 0° of a column of mercury 
1 metre long and 1 square millimetre in section. 
Our own experiments lead us to a value not differing much from that of Siemens. 
We find 
1 mercury uni t='95418 B.A. unit. 
If we assume that the B.A. unit is '98651 ohm (in accordance with our deter¬ 
mination), we find 
1 mercury unit=‘94130 ohm, 
the ohm being 10 9 C.G.S. The same result may be expressed in another way by 
saying that the ohm is the resistance of a column of mercury at 0°, 1 square millimetre 
in section, and 1062'4 millims. in length. 
Through the kindness of Dr. C. W. Siemens we have had an opportunity of 
comparing with the B.A. units a standard mercury unit (No. 2513) issued by 
Messrs. Siemens and Halske. At the proper temperature (16°*7) we find that its 
resistance is 
'95365 B.A. unit, 
agreeing very closely with previous comparisons of Siemens’ mercury measurements 
with the B.A. unit. 
The determination of the specific resistance of mercury is simple enough in principle, 
though the execution is somewhat tedious, and the calculation of the results is com¬ 
plicated in practice by the necessity of introducing various temperature corrections. 
In a first sketch of the method it will be convenient to omit these corrections, which 
is tantamount to supposing that all the measurements are made at zero. If L be the 
length and s the section of the column of mercury, B its resistance, r the specific 
resistance of the metal, 
R=—, or r=Rf 
S L 
The length L can be measured directly, but s can only be found with the necessary 
accuracy from the contents. Thus if p be the specific gravity of mercury, and W the 
weight of the whole column in grammes, pLs=W, whence s=W/pL, and 
RW 
Apart from the temperature corrections already referred to, the simplicity of the 
formula is disturbed by the inevitable departure from the truly cylindrical form of the 
glass tubes used to contain the mercury. It is true indeed that to a first order of 
approximation the formula stands unaltered, as we may see if we understand by s the 
mean section of the tube. The volume is still truly expressed by -sL, and the resist¬ 
ance is approximately expressed by rL/s. If, however, the squares of the variations 
