Now I want particularly to ask you to guard against the idea that all
these results obtained from electromagnets are equally applicable to
permanent magnets of steel; they are not, for this simple reason. With
an electromagnet, when you put the armature near, and make the
magnetic circuit better, you not only get more magnetic lines going
through that armature, but you get more magnetic lines going through
the whole of the iron. You get more magnetic lines round the bend when
you put an armature on to the poles, because you have a magnetic
circuit of less reluctance with the same external magnetizing power in
the coils acting around it. Therefore, in that case, you will have a
greater magnetic flux all the way round. The data obtained with the
electromagnet (Fig. 42), with the exploring coil, C, on the bend of
the core, where the armature was in contact, and when it was removed
are most significant. When the armature was present it multiplied the
total magnetic flow tenfold for weak currents and nearly threefold for
strong currents. But with a steel horseshoe, magnetized once for all,
the magnetic lines that flow around the bend of the steel are a fixed
quantity, and, however much you diminish the reluctance of the
magnetic circuit, you do not create or evoke any more. When the
armature is away the magnetic lines arch across, not at the ends of
the horseshoe only, but from its flanks; the whole of the magnetic
lines leaking somehow across the space. Where you have put the
armature on, these lines, instead of arching out into space as freely
as they did, pass for the most part along the steel limbs and through
the iron armature. You may still have a considerable amount of
leakage, but you have not made one line more go through the bent part.
You have absolutely the same number going through the bend with the
armature off as with the armature on. You do not add to the total
number by reducing the magnetic reluctance, because you are not
working under the influence of a constantly impressed magnetizing
force. By putting the armature on to a steel horseshoe magnet you
only _collect_ the magnetic lines, you do not _multiply_ them. This is
not a matter of conjecture. A group of my students have been making
experiments in the following way: They took this large steel horseshoe
magnet (Fig. 52), the length of which, from end to end, through the
steel, is 42½ 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. 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.