344 Dr Searle, The comparison of nearly 
these wires connexions are made to the battery, the galvanometer 
and the resistance boxes supplying the shunts, coupling screws 
being used. A more convenient plan is to fit the copper pieces 
with proper terminal screws. The four coils A, B, C, D are 
inserted in the gaps as shown diagrammatically in Fig. 3. 
The resistance of the part of the copper connector OL, which 
lies between the coil C and the point where the battery wire is 
attached to L, together with the resistance of the part of P, which — 
lies between C and the point where the galvanometer wire is 
attached to P, counts as part of C itself and is therefore eliminated | 
in the equations. Similar remarks apply to the copper connectors” 
which are joined to the coil D. The effect of the finite resistances _ 
of the connectors joined to A and B is discussed in § 9 and is 
shown to be negligible in practice. ; 
To prevent undue heating of the coils, a sufficiently great 
resistance R should be placed in series with the battery. The 
galvanometer should be permanently connected to M and P, and a 
tapping key K should be placed in the battery circuit. In this 
way errors due to thermoelectric effects are avoided. The in- 
ductances of the coils are too small to give rise to trouble on 
making or breaking the battery connexion. 
In order that, after a balance has been obtained, it should not 
be upset by changes in the resistances of the coils due to rise of 
temperature brought about by the passage of the current, it is 
necessary that the coil A should be similar to the coil B and that ~ 
the coil C should be similar to the coil D. When the bridge is 
arranged as in Figs. 1 and 2, this similarity secures such constancy 
of the ratios A/B and C/D that the passage of the current does 
not upset the balance when once it has been obtained. 
The “Sub-standards” of resistance (Fig. 4) supplied by 
Mr R. W. Paul have proved very suitable for the experiment. 
These coils are wound with wire of small temperature coefficient 
and are well ventilated so that they carry comparatively large 
currents without serious rise of temperature. Using four of these 
