I shift the exploring coil,
right up to the bend; replace the armature; wait until the spot of
light is brought to rest at the zero of the scale. Now, on detaching
the armature, the movement of the spot of light is quite
imperceptible. In our careful laboratory experiments, the effect was
noticed inch by inch all along the magnet. The effect when the
exploring coil was over the bend was not as great as 1-3000th part of
the effect when the coil was hard up to the pole. We are, therefore,
justified in saying that the number of magnetic lines in a permanently
magnetized steel horseshoe magnet is not altered by the presence or
absence of the armature.
You will have noticed that I always put on the armature gently. It
does not do to slam on the armature; every time you do so, you knock
some of the so-called permanent magnetism out of it. But you may pull
off the armature as suddenly as you like. It does the magnet good
rather than harm. There is a popular superstition that you ought never
to pull off the keeper of a magnet suddenly. On investigation, it is
found that the facts are just the other way. You may pull off the
keeper as suddenly as you like, but you should never slam it on.
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