]
It is interesting to note how the perfecting of the magnetic circuit
increases the self-induction.
Thanks to the kindness of Mr. Preece, I have been furnished with some
most valuable information about the coefficients of self-induction,
and the resistance of the standard pattern of relays, and other
instruments which are used in the British postal telegraph service,
from which data one is able to say exactly what the time constants of
those instruments will be on a given circuit, and how long in their
case the current will take to rise to any given fraction of its final
value. Here let me refer to a very capital paper by Mr. Preece in an
old number of the "Journal of the Society of Telegraph Engineers," a
paper "On Shunts," in which he treats this question, not as perfectly
as it could now be treated with the fuller knowledge we have in 1890
about the coefficients of self-induction, but in a very useful and
practical way. He showed most completely that the more perfect the
magnetic circuit is--though of course you are getting more magnetism
from your current--the more is that current retarded. Mr. Preece'e
mode of experiment was extremely simple. He observed the throw of the
galvanometer when the circuit which contained the battery and the
electromagnet was opened by a key which at the same moment connected
the electromagnet wires to the galvanometer.
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