Distance between
shoes.            Attraction,
Millimeters.       in grammes.

2                  900
10                1,012
15                1,025
25                  965
40                  890
60                  550

[Footnote 2:  “La Lumiere Electrique,” vol. iv., p. 129.]

With a stronger battery the magnet without shoes had an attraction of 885 grammes, but with the shoes 15 millimeters apart, 1,195 grammes.  When one pole only was employed, the attraction, which was 88 grammes without a shoe, was diminished by adding a shoe to 39 grammes!

CONTRAST BETWEEN ELECTROMAGNETS AND PERMANENT MAGNETS.

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: