326 H. A. Rowland—Absolute Unit of Electrical Resistance. 
Hence, to measure the absolute resistance of a circuit on this 
method, we must calculate M and measure the ratio of Q to E. 
M is known as the mutual potential of the two circuits with 
unit currents, and mathematical methods are known for its 
calculation. 
The simplest and best form in which the wire can be wound 
for the calculation of M is in parallel circular coils of equal 
size and of as small sectional area as possible. For measuring 
E a tangent galvanometer is needed, and we shall then have 
B= tan 6, 
where H is the horizontal intensity of the earth’s magnetism at 
the place of the tangent galvanometer, and G@ the constant of 
the galvanometer. 
or measuring Q we must use the ballistic method, and we 
ve 
A cid 
Biot Ei ge tT! Ig. avaer 
hse aad EES 2 sin $0 
Qa = che ‘2 sin $0, 
which for very small values of A becomes 
be ee 
Ne eg GO antand 1 
Te G Tsin$@1+4A—4A2’ 
where H’ is the horizontal component of the earth’s magnetism 
at the place of the small galvanometer, G’ its constant, T 
the time of vibration of the needle, and A the logarithmic 
decrement. ' 
The ratio of H’ to H can be determined by allowing a needle 
to vibrate in the two positions. But this introduces error, and 
by the following method we can eliminate both this and the 
distance of the mirror from the scale by which we find 6 and 
eter, before and after each experiment. Let a and a’ be the 
deflections of the tangent galvanometer and the other galva- 
be 
— tan a= ae tana’, 
and we have finally 
\>7 G tana’ tané 
R=Mz 
TG’ tana sing# 1445-42? 
