equal electrical resistances. 341 
many advantages both as regards accuracy and as regards the in- 
struction of students that it may be useful to other teachers to 
give an account of the method as employed in my practical class 
at the Cavendish Laboratory. 
§ 2. General theory. The theory of the method is as follows: 
Let C, D(Figs. 1, 2) be two nearly equal resistance coils; in practice 
they would not differ by as much as one part in 1000. 
Fig. 1. 
The two coils A, B, which are to be compared, are first con- 
nected with the coils O, D to form the four arms of a Wheatstone’s 
bridge, as in Fig. 1, the exact balance being obtained by shunting 
A with a high resistance a, and B with a high resistance b,. The 
coils A and B are then interchanged, so that they are now arranged 
as in Fig. 2, and the balance is obtained by shunting A with a, 
and B with by. 
It will not, as a rule, be necessary to shunt both A and B at 
the same time*, so that two of the four resistances a, a2, b,, b, 
will be infinite. But it will be convenient to consider the mathe- 
matics of the problem without this restriction. 
Which of the two coils A and B will require shunting in 
either of the two arrangements will depend upon the relative 
values of A, B, C, D. The four possible cases are as follows: 
A shunted in both arrangements. 
B shunted in both arrangements. 
A shunted in the first arrangement, B shunted in the second 
arrangement. 
B shunted in the first arrangement, A shunted in the second 
arrangement. 
* Tf no very high resistances are available, it may be necessary to apply shunts 
to both A and B in order to secure a satisfactory balance. 
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