Take from an ordinary relay a coil, with its iron core, half the
electromagnet, so to speak, without any yoke or armature. Connect it
up as described, and observe the throw given to the galvanometer. The
amount of throw obtained from the single coil was taken as unity, and
all others were compared with it. If you join up two such coils as
they are usually joined, in series, but without any iron yoke across
the cores, the throw was 17. Putting the iron yoke across the cores,
to constitute a horseshoe form, 496 was the throw; that is to say, the
tendency of this electromagnet to retard the current was 496 times as
great as that of the simple coil. But when an armature was put over
the top, the effect ran up to 2,238. By the mere device of putting the
coils in parallel, instead of in series, the 2,238 came down to 502, a
little less than the quarter value which would have been expected.
Lastly, when the armature and yoke were both of them split in the
middle, as is done in fact in all the standard patterns of the British
postal telegraph relays, the throw of the galvanometer was brought
down from 502 to 26. Relays so constructed will work excessively
rapidly. 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. In long pieces of iron the mutual action of the
various parts tends to keep in them any magnetization that they may
possess; hence they are less readily demagnetized. In short pieces,
where these mutual actions are feeble or almost absent, the
magnetization is less stable, and disappears almost instantly on the
cessation of the magnetizing force. Short bits and small spheres of
iron have no magnetic memory. Hence the cause of the commonly received
opinion among telegraph engineers that for rapid work electromagnets
must have short cores. As we have seen, the only reason for employing
long cores is to afford the requisite length for winding the wire
which is necessary for carrying the needful circulation of current to
force the magnetism across the air gaps. If, for the sake of rapidity
of action, length has to be sacrificed, then the coils must be heaped
up more thickly on the short cores. The electromagnets in American
patterns of telegraphic apparatus usually have shorter cores, and a
relatively greater thickness of winding upon them, than those of
European patterns.
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