REVIEW 467

total field flux pulsates. The reaction machine has low output
and low power-factor, but the type is useful in small synchronous
motors, due to the simplicity resulting from the absence of direct-
current field excitation.

Rectifiers.—XV, 138. Apparatus to convert alternating into
direct current by synchronously changing connections. Rec-
tification may occur either by synchronously reversing connec-
tions between alternating-current and direct-current circuit:
reversing rectifier, or by alternately making contact between the
direct-current circuit and the alternating-current circuit, when
the latter is of the right direction, and opening contact, when
of the reverse direction: contact-making rectifier. Mechanical
rectifiers may be of either type. Arc rectifiers, such as the mer-
cury-arc rectifier, which use the unidirectional conduction of the
arc, necessarily are contact-making rectifiers.
, Full-wave rectifiers are those in which- the direct-current cir-
cuit receives both half waves of alternating current; half-wave
rectifiers those in which only alternate half waves are rectified,
the intermediate or reverse half waves suppressed. The latter
type is permissible only in small sizes, as the interrupted pul-
sating current traverses both circuits, and produces in the alter-
nating-current circuit a unidirectional magnetization, which
may give excessive losses and heating in induction apparatus.
The foremost objection to the mechanical rectifier is, that the
power which can be rectified without injurious inductive spark-
ing, is limited, especially in single-phase rectifiers, but for small
amounts of power, as for battery charging and constant-current
arc lighting they are useful. However, even there the arc recti-
fier is usually preferable. The brush arc machine and the
Thomson Houston arc machine were polyphase alternators with
rectifying commutators.

Regulating Pole Converter.—Variable-ratio converter., Split-
pole converter, XXI, 230. A synchronous converter, in which
the ratio between direct-current voltage and alternating-current
voltage "can be varied at will, over a considerable range, by shift-
ing the direction of the resultant magnetic field flux so that the
voltage between the commutator brushes is less than maximum
alternating-current voltage, and by changing, at constant im-
pressed effective alternating voltage, the maximum alternating-
current voltage and with it the direct-current voltage, by the
superposition of a third harmonic produced in the converter in

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