They took this large steel horseshoe magnet (Fig. 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.  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.