ed on. So then it
comes to a question of the length of time for which you want to work
it. What fraction of a second do you require your signal to be given
in? What is the rate of the vibrator of your electric bell? Suppose
you have settled that point, and that the short time during which the
current is required to rise is called t; then the apparent resistance
at time t after the current is turned on is given by the formula:

R_{t} = R x e^{(R/L)t} + ( e^{(R/L)t} - 1 )

TIME CONSTANTS OF ELECTROMAGNETS.

I may here refer to some determinations made by M. Vaschy,[1]
respecting the coefficients of self-induction of the electromagnets of
a number of pieces of telegraphic apparatus. Of these I must only
quote one result, which is very significant. It relates to the
electromagnet of a Morse receiver of the pattern habitually used on
the French telegraph lines.

L, in quadrants.
Bobbins, separately, without iron cores. 0.233 and 0.265
Bobbins, separately, with iron cores. 1.65 and 1.71
Bobbins, with cores joined by yoke,
coils in series 6.37
Bobbins, with armature resting on poles. 10.68

[Footnote 1: "Bulletin de la Societe Internationale des Electriciens,"
1886.]

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 si