: _.- - : 2 IN PARALLEL
| . :. - :
| . / : - :
| . / - :
|. / - : :
|./. : :
|/_____:_____________:____________________________ t
10 20 40 60 80 100 120
FIG. 54.--CURVES OF RISE OF CURRENTS.
This definite fraction is the fraction (e - 1)/e; or in decimals,
0.634. All curves of rise of current are alike in general shape, they
differ only in scale, that is to say, they differ only in the height
to which they will ultimately rise, and in the time they will take to
attain this fraction of their final value.
_Example (1)._--Suppose E = 10; R = 200 ohms; L = 8. The final value
of the current will be 0.025 amp. or 25 milliamperes. Then the time
constant will be 8 / 400 = 1-50th sec.
_Example (2)._--The P.O. Standard "A" relay has R = 400 ohms; L =
3.25. It works with 0.5 milliampere current, and therefore will work
with 5 Daniell cells through a line of 9,600 ohms. Under these
circumstances the time constant of the instrument on short circuit is
0.0081 sec.
It will be noted that the time constant of a circuit can be reduced
either by diminishing the self-induction or by increasing the
resistance.
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