equal electrical resistances. 345 
sub-standards for A, B, C, D, in Figs. 1 and 2, the students at the 
Cavendish Laboratory are able to make reasonably good com- 
parisons in a room where there is much vibration due to moving 
machinery, the galvanometer being a table instrument having a 
pointer moving over a divided scale. 
The results given below are not intended to illustrate the 
power of the method when used under favourable conditions; they 
are intended rather to show how well the method works under 
unfavourable conditions. 
§ 4. Practical example. The following results obtained by 
G. F.C. Searle and A. L. Hughes will illustrate the working of 
the method. Four “sub-standards” A, B, C, D, each of nominally 
one ohm resistance, were employed. 
Position 1. 
Coil A not shunted. Hence a,=0 ohms and 1/a,;=0ohm~!. 
Coil B shunted with 2800 ohms. Hence 1/b,=0:000357 ohm7~!. 
Position 2. 
Coil A not shunted. Hence a,=00 ohms and 1/a,=0 ohm7~1. 
Coil B shunted with 4800 ohms. Hence 1/b,=0:000208 ohm7~!. 
Hence, using (6), 
eee Sealy ally 
Al QD\ GR Gy BIB NG Boy 
we have 
oe le soil 1 
Fah + 9 (0000357 + 0:000208) = Bt 0000282 ohm~}, 
or B-—-A=ABx 0:000282 ohm. 
The difference between A and B is so small, and each is so nearly one ohm, 
that we may put 46=1 ohm? and thus obtain 
B—A=0-000282 ohm. 
§5. Intercomparison of three coils. A useful test of the 
accuracy of the method is obtained if three coils A, B, C are used. 
First A and B are compared as described in §§ 2, 3, using C and D 
as the auxiliary coils whose ratio is eliminated. Then A is com- 
pared with C, using B and D as the auxiliary coils. Finally B is 
compared with C, using A and D as the auxiliary coils. In this 
way the values of the three differences 
TBA AL ot! Nh BN=LC 
are found. The accuracy of the work may be tested by comparing 
the value of B—O found directly with that found from the two 
differences A—Band A—C. The shunts on C may be denoted 
I Ci, Ca. 
