236

ELECTRICAL APPARATUS

ever, from which the direct current, ?"o, is derived by reversal, must
go through zero twice during each period, thus must have the
shape shown as i in Fig. 76, that is, must abruptly reverse. If,
however, the supply circuit contains any self-inductance—and
every circuit contains some inductance—the current can not
change instantly, but only gradually, the slower, the higher the
inductance, and the actual current in the supply circuit assumes

FIG. 77.—Differential current on rectifier on inductive load.

a shape like that shown in dotted lines in Fig. 76. Thus the cur-
rent in the alternating part and that in the rectified part of the
circuit can not be the same, but a difference must exist, as shown
as i1 in Fig. 77. This current, if, passes between the two parts

AC

FIG. 78.—Rectifier with A.C. and D.C. shunt resistance for inductive load
of the circuit, as arc at the rectifier brushes, and causes the recti-
fying commutator to spark, if there is any appreciable inductance
in the circuit. The intensity of the sparking current depends
on the inductance of the rectified circuit, its duration on that of
the alternating supply circuit.
By providing a byepath for this differential current, if, the
sparking is mitigated, and thereby the amount of power, which can
be rectified, increased. This is done by shunting a non-inductive
resistance across the rectified circuit, r0, or across the alternating
circuit, r, or both, as shown in Fig. 78. If this resistance is low,
it consumes considerable power and finally increases sparking