| 5 | 1.649 | 3.936 |
| 10 | 2.718 | 6.343 |
| 20 | 7.389 | 8.646 |
| 30 | 20.08 | 9.501 |
| 60 | 403.4 | 9.975 |
| 120 | 16200.0 | 9.999 |
----------------------------------------

In this case the value of the steady current as calculated by Ohm's
law is 10 amperes, but Helmholtz's law shows us that with the great
self-induction which we have assumed to be present, the current, even
at the end of 30 seconds, has only risen up to within 5 percent. of
its final value; and only at the end of two minutes has practically
attained full strength. These values are set out in the highest curve
in Fig. 54, in which, however, the further supposition is made that
the number of spirals, S, in the coils of the electromagnet is 100, so
that when the current attains its full value of 10 amperes, the full
magnetizing power will be Si = 1000. It will be noticed that the
curve rises from zero at first steeply and nearly in a straight line,
then bends over, and then becomes nearly straight again, as it
gradually rises to its limiting value. The first part of the
curve--that relating to the strength of the current after _very small_
interval of time--is the period within which the strength of the
current is governed by inertia (i.e., the self-induction) rather than
by resistance. In reality the current is not governed either by the
self-induction or by the resistance alone, but by the ratio of the
two. This ratio is sometimes called the "time constant" of the
circuit, for it represents _the time_ which the current takes in that
circuit to rise to a definite fraction of its final value.

E = 10
r = 1
R = 100
L = 10

Si
1000 + _..-------------------------------
| . _ _---------
| . .----
| . .- 2 IN SERIES
| . .-
| -
| .: - :
| .: . :
500 | . : __- -:---------------------------
| . : _.- - : 2 IN PARALLEL
| . :. - :
| . / : - :
| . / - :
|. / - : :
|./. : :
|/_____:_____________:____________________________ t
10