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. 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.

From these experimental results I pass to the special design of
electromagnets for special purposes.

ELECTROMAGNETS FOR MAXIMUM TRACTION.

These have already been dealt with in the preceding lecture; the
characteristic feature of all the forms suitable for traction being
the compact magnetic circuit.

Several times it has been proposed to increase the power of
electromagnets by constructing them with intermediate masses of iron
between the central core and the outside, between the layers of
windings. All these constructions are founded on fa