Mr. Preece states that with the old pattern of relay having
so much self-induction as to give a galvanometer throw of 1,688, the
speed of signaling was only from 50 to 60 words per minute, whereas,
with the standard relays constructed on the new plan, the speed of
signaling is from 400 to 450 words per minute. It is a very
interesting and beautiful result to arrive at from the experimental
study of these magnetic circuits.
SHORT CORES _versus_ LONG CORES.
In considering the forms that are best for rapid action, it ought to
be mentioned that the effects of hysteresis in retarding changes in
the magnetization of iron cores are much more noticeable in the case
of nearly closed magnetic circuits than in short pieces.
Electromagnets with iron armatures in contact across their poles will
retain, after the current has been cut off, a very large part of their
magnetism, even if the cores be of the softest of iron. But so soon as
the armature is wrenched off, the magnetism disappears. An air gap in
a magnetic circuit always tends to hasten demagnetizing. A magnetic
circuit composed of a long air path and a short iron path demagnetizes
itself much more rapidly than one composed of a short air path and a
long iron path.
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