224 
MESSRS. R. T. GLAZEBROOK AND J. M. DODDS ON THE 
correction which Howland had to apply for the difference in the intensity of the 
horizontal component of the earth’s magnetic force at the two galvanometers. 
We have instead to determine exactly the ratio between two resistances of about 
1 and 3000 B.A. units respectively, and to show that the heating of the wires by the 
current could never be such as to affect the value of this ratio appreciably. We 
proceed to describe the arrangement of the apparatus. 
Fig. 1 gives a diagrammatic plan. A and B are the two coils, A being the primary. 
G is the galvanometer. K, L, M, N are four mercury cups. 
P, Q, H, and II' are also mercury cups. C is the commutator, and D the battery. 
Between H' and G is a resistance, S, of about 3000 B.A. units, between H and L a 
second resistance, V, of about 1 unit. 
Between P and M there is an adjustable resistance, T, the purpose of which will be 
described shortly. 
The rest of the figure represents the ordinary Wheatstone’s bridge arrangement 
for measuring the resistance of the secondary circuit. P', Q' are two mercury cups. 
E, F a divided wire. B a resistance of about 160 B.A. units (the total resistance of 
the secondary circuit). U and W two equal resistances of about 30 units, forming 
the other arms of the bridge. 
Let us suppose P P', Q Q', and M N are connected by stout copper j -| - shaped 
pieces. Then our secondary circuit, broken between P and Q, forms the fourth arm 
of the Wheatstone’s bridge, and by adjusting the variable resistance, T, the resis¬ 
tance of the secondary circuit can be made so nearly equal to B that the difference 
between them may be expressed in terms of the resistance of the bridge wire in the 
ordinary manner. 
B then forms our standard resistance, and is the quantity actually measured in the 
