340 Dr Searle, The comparison of nearly 
The comparison of nearly equal electrical resistances. By 
G. F. C. SEARLE, Se.D., F.R.S., University Lecturer in Experi- 
mental Physics, Fellow of Peterhouse. | 
[Head 24 November 1913.] 
§1. Introduction. The ratio of the resistances of two very 
nearly equal coils can be determined most accurately by finding 
the very small difference between their resistances. ‘The method, 
which, perhaps, is most widely known, is that of Carey Foster. 
In that method the difference is expressed as the resistance of — 
a measured length of the graduated wire of the Carey Foster 
bridge. It is therefore necessary to know the resistance of each 
centimetre of this wire and to have a table of calibration corrections, 
since it 1s impossible to procure an absolutely uniform wire. Even 
if the wire were initially uniform, it would by use become non- 
uniform through the wear caused by the contact of the sliding 
contact piece. 
In the bridge designed by Dr J. A. Fleming and used for 
many years at the Cavendish Laboratory by Dr R. T. Glazebrook 
and others in the comparison of resistance coils with the standards 
of the British Association, the wire is about 1 metre in length and 
has a resistance of about 1/20 ohm. Thus to measure a difference 
of resistance of 1/200,000 ohm a movement of the contact piece. 
of 1/10 mm. has to be observed and measured. 
In recent years the use of Carey Foster's method has been 
abandoned for resistance comparisons of the highest precision and 
a method of shunting is now employed. This involves the use of 
a resistance box capable of furnishing high resistances, and, in 
strictness, the coils in this box should be compared with a standard 
resistance. But the resistance boxes now supplied by any good 
instrument maker are so accurately adjusted that it is quite un- 
necessary to calibrate them if they are only to be used as shunts, 
The great advantage of the method of shunting is that instead 
of dealing with the resistance of one or two millimetres of the 
wire of the bridge—a length which could not be easily read to 
less than 5 mm.—we have to deal with shunting resistances 
measured by many hundreds or thousands of ohms, these re-— 
sistances not differing from their nominal values by as much as 
one part in a thousand, “if the resistance box has been well 
adjusted. 
The method of shunting has been in use for some years at the - 
National Physical Laboratory and in other standardising labora- 
tories, but it has hardly made its way into the laboratory courses 
intended for elementary students of physics. It has, however, so 
