ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 43 
ment himself. We can state with the very highest exactness that the 
resistance of the coil X at a temperature A° C. is R. To obtain the tem- 
perature coefficient accurately requires an amount of labour which may 
be quite unnecessary for the purpose for which the coil is to be used. 
But it is requisite to have standards of higher value than one unit, 
and part of the Association grant has been used in obtaining coils of a 
resistance of 10, 100, 10,00 and 100,00 units. Two of each value have 
been purchased, so that by frequent comparison of one with the other 
any accident to either may be checked. 
It remains, therefore, to describe how these coils are to be referred to 
the standards. For the 10 units two methods have been adopted. 
There are at the Cavendish Laboratory two five-unit coils. Each of 
these was compared with five single units placed in series, using Fleming’s 
bridge to make the comparison, and the ten-unit coil was compared with 
these two in series. 
The values obtained by two observers at a temperature of 12° were :— 
9°98360 . : Z . ; ; . Lord Rayleigh. 
9:98393 . , : : ; . wa. hte hs. Gre 
For the second method, suppose we have three coils each of resistance 
about 3 units. Let there be 3 + a, 3 + 6 and 3 + y, then the resistance 
of the three in series is 9 + a + § + y, and in multiple arc, if we neglect 
terms like a? J, &c.,itis 1 +4 (a+ + vy), thus neglecting terms such 
as a? 1, the resistance of the three in series is just 9 times that of the three 
in multiple are. j 
But the three coils in multiple are are very nearly one unit, and can 
be compared with the standards. If then we combine in series with the 
same three one of the standards we have a resistance of approximately 
ten units, whose value is very accurately known, and with which any 
other ten-unit coil can be compared by the aid of Fleming’s bridge. 
Lord Rayleigh has devised an arrangement of mercury cups, by means 
of which the changes indicated can be easily performed. 
The three 3-unit coils are wound on the same bobbin, and inclosed in 
the same case. The six electrodes project in pairs, and their ends lie 
in a plane. The figure represents a piece of ebonite, through which, 
holes are cut as indicated by the letters a, b, &e. 
