52), the length of which, from end to end, through the
steel, is 421/2 inches. A light, narrow frame was constructed so that it
could be slipped on over the magnet, and on it were wound 30 turns of
fine wire, to serve as an exploring coil. The ends of this coil were
carried to a distant part of the laboratory, and connected to a
sensitive ballistic galvanometer. The mode of experimenting is as
follows:
The coil is slipped on over the magnet (or over its armature) to any
desired position. The armature of the magnet is placed gently upon the
poles, and time enough is allowed to elapse for the galvanometer
needle to settle to zero. The armature is then suddenly detached. The
first swing measures the change, due to removing the armature, in the
number of magnetic lines that pass through the coil in the particular
position.
[Illustration: FIG. 52.--EXPERIMENT WITH PERMANENT MAGNET.]
I will roughly repeat the experiment before you: The spot of light on
the screen is reflected from my galvanometer at the far end of the
table. I place the exploring coil just over the pole, and slide on the
armature; then close the galvanometer circuit. Now I detach the
armature, and you observe the large swing.
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