118 E. H. Hall—Rotation of the Equipotential Lines 
strip was placed between the poles P, P, of the electromagnet. 
The tank was filled with water which was sometimes at rest 
and sometimes flowing. By this means the temperature of the 
t 
sened. The diameter of the plane cireular ends of the pole 
pieces P, P, is about 3-7 cm. 
The general method of most of the experiments to be men- 
tioned did not differ much from that described in this Souris 
for September, 1880. The intensity of the magnetic field was 
estimated as before, by the impulse given to a galvanometer 
needle when a small coil in connection with the galvanometer 
was suddenly removed from the field. This impulse was com 
pared with that given to the same needle by the current 
obtained by turning an earth inductor of known dimensions. 
The direct current through the strip under examination was 
measured by means of a tangent galvanometer. The transverse 
ate on the transverse cu 
The rotational powers mak then, be given in ostensibly 
absolute measure, but an uncertainty of several per cent 
