Intensities of 'Powerful Magnetic Fields. 149 
attachment of the cord above until the wire was exactly oppo- 
site two marks, one at the upper end and one at the lower 
end of the vertical central line of the pole-face. A thin 
thread, t 1} was attached to the wire just above the electro- 
magnet, and carried out to one side nearly horizontally, and 
attached by its other end at a point near the lower end of 
the cord of the pendulum, P x . A second thread, t 2 , was 
attached to the lower end of the wire, and carried out parallel 
to the thread t 1} and attached to a second pendulum, P 2 , at a 
somewhat greater distance from the wire. The pendulums, 
Pi, P 2 , were simply weights, in some cases of half a pound 
each, and in other cases 100 grammes each, hung by fine 
cords from one to two metres long from blocks, p 1} p 2} 
which could be moved along a horizontal scale, Si, placed 
nearly in a vertical plane passing through the wire at right 
angles to the coil of the electromagnet, and therefore parallel 
to the pole-face. A second scale, S 2 , placed just above the 
point of attachment of the upper thread, and nearly in the 
same vertical plane as S 1? gave the position at any time of a 
point in each of the pendulum-cords at the same distance 
below the point of suspension. 
The method of performing an experiment was as follows: — 
The vertical positions of the cords of the pendulums when the 
threads £ l5 1 2 were quite slack and the wire was in position were 
read off from the scales. A current was then sent through the 
wire and coil in series. The electromagnetic action between the 
current in the wire and the magnetic field caused the wire to 
move bodily to one side across the lines of force; and the direc- 
tion of the current in the wire was so arranged that the side 
towards which the wire moved was that remote from the pen- 
dulums. By moving out the blocks, p 1} p 2 , pull (in conse- 
quence of the deviation of the pendulums from the vertical) 
was brought to bear through the threads t 1} t 2 on the upper 
and lower ends of the wire; and this was gradually increased 
until the wire was brought back as nearly as possible to 
its former vertical position. The readings given by the 
pendulums on the two scales were then taken, and at the same 
time a reading of the current-strength on the current-galvano- 
meter. The difference between the two motions on Si and S 2 
for either pendulum, divided by the distance between the 
scales, gave the tangent of the inclination of the pendulum to 
the vertical. 
Let, now, I be the mean intensity for the position of the 
wire of the magnetic field in C.Gr.S. units, 7 the current flow- 
ing on the wire also in C.G.S., L the length of the wire in 
the field in centims., F the force per unit of length on the wire 
in dynes, we have ]/y = F. 
