Intensities of Powerful Magnetic Fields. 151 
position in the direction to cause the siphon to press against 
the thread. The point of suspension of the pendulum was 
then moved so as to restore the coil to its initial position, and 
the distance through which it was moved noted. This dis- 
tance, divided by the vertical height of the point of suspension 
above the point at which the thread touched the siphon, gave 
the tangent of the inclination of the thread to the vertical; and 
this multiplied by the downward pull in dynes on the weight 
gave the horizontal force restoring the coil to its equilibrium 
position. 
When equilibrium had been obtained, the difference of 
potentials between the terminals of the coil was immediately 
measured in volts by means of a potential-galvanometer pre- 
viously arranged in readiness; and from the result and the 
known resistance of the coil the current flowing was deduced 
in C.Gr.S. units. The mean intensity I of the field over the 
space occupied by the coil in the equilibrium position was then 
easily calculated. For let y be the current in the coil, 6 the 
inclination of the pendulum-thread above the siphon to the 
vertical, I the distance in centimetres of the point at which 
the thread touched the siphon from the vertical axis round 
which the coil turned, and W the pendulum-weight in 
grammes. Using L and b with the same meaning as before ; 
we have plainly the equation of equilibrium 
IL& 7 = "W>Ztan0 ; 
and therefore "Wnl 
l -T$}**? («) 
The method of determining a magnetic field-intensity by 
measuring the electromagnetic action on a conductor carrying 
a current in the field, may be very conveniently applied to the 
space between the opposite poles of a field magnet in which 
the armature-coils of a dynamo are made to revolve. A rect- 
angular frame of stout copper wire or narrow copper strip, 
containing one or more turns (insulated from one another if 
there are more than one), is constructed of such a size that 
two opposite sides are at a distance apart equal to the dia- 
meter of the armature, and of length equal to the length of 
the pole-faces. The rectangle should begin and end near the 
middle of the two other sides; and the two ends of the wire 
should be brought out side by side, insulated from one another^ 
at right angles to the side, a distance of two or three inches. 
A pair of knife-edges, which may be made for ease of a piece 
of hardened copper or brass wire filed so as to have a nearly 
triangular section, are soldered, one at the centre of the side 
where the ends of the wire are brought out, the other at the 
centre of the opposite side ; so that the rectangle is symme- 
