S YNCHRONO US RECTIFIER 255

rectifying commutators are connected in series, as shown in
Fig. 119. This represents two quarter-phase rectifiers in series
displaced from each other by 45°, that is, an eight-phase system.
Three-phase star-connected rectification, Fig. 106, has been
used in the Thomson-Houston arc machine, and quarter-phase
rectification, Fig. 108, in the Brush arc machine, and for larger
powers, several such commutators were connected in series, as
in Fig. 119. These machines are polyphase (constant-current)

FIG. 120.—Counter e.m.f. shunting gaps of six-phase rectifier.
alternators connected to rectifying commutators on the armature
shaft.
For a more complete discussion of the rectification of arc
machine see " Theory and Calculation of Transient Electric
Phenomena/7 Section II.
145. Even with polyphase rectification, the power which can
be rectified is greatly limited by the sparking caused by the dif-
ferential current, that is, the difference between the rectified
current, i0, which never reverses, but is practically constant, and
the alternating supply current. Resistances shunting the gaps
between adjoining segments, as byepath for this differential cur-
rent, consume power and mitigate the sparking to a limited extent
only. A far more effective method of eliminating the sparking
is by shunting this differential current not through a mere non-
inductive resistance, but through a non-inductive resistance which
contains an alternating counter e.m.f. equal to that of the supply
phase, as shown diagrammatically in Fig. 120.
In Fig, 120, ei to e6 are the six phases of a ring-connected six-
phase system; e\ to e\ are e.m.fs. of very low self-inductance