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.
* * * * *
ELECTRIC ERYGMASCOPE.
The erygmascope is the name of an electric lighting apparatus designed
for the examination of the strata of earth traversed by boring
apparatus.
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