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.
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