A. M. Mayer—Researches in Electro-Magnetism. 197 
west; at C the compass; and at G the tangent galvanometer | 
sufficiently removed from E and W, not to be affected by the 
helices. : 
It was found that when the needle was placed at such a posi- 
tion on the line between the helices that it stood at 0° when the 
intensity of action of the electro-magnet on the needle with a 
change of its distance from the magnetized core, we will have a 
system of measurement for the electro-magnetic forces which 
will exceed in in delicacy, though be similar in 
arrangement, to the best photometric processes. 
The advantages of this method, as far as my knowledge of it 
extends, is that you can thus subject the needle to two opposing 
actions of great intensity and thus any minute difference in the 
causes which alter their relative intensities will, on account of 
e law of variation of the intensity of the force with a change of 
distance.-—To find the law of variation of the force with the dis- 
tance from the pole of the core (which as is well known exists 
in the plane of the end-coils of the helix) the following experi- 
ments were made. 
A core 10 in. long and 1°64 in. in diameter composed of 400 
soft well annealed iron wires, was placed in helix E and the 
plane of the ends of the wires facing the needle were brought 
into the plane of the end coils of the helix. The center of the 
compass needle was placed at distances of 4, 5, 6, 7 and 8 ft. 
from the end of the core; the needle brought to 0° at eath of 
these positions by the action of terrestrial magnetism and the 
current passed from seven new, freshly amalgamated Bunsen- 
cells; as soon as the needle had come to rest the angle of deflec- 
tion was read and at the same time the tangent galvanometer 
was noted. 
