REACTION MACHINES 273
impressed voltage, but the relative position of the rotor with
regards to the phase of the impressed voltage is more accurately
maintained. Where this feature is of importance, as in driving
a contact-maker, a phase indicator or a rectifying commutator,
the reaction machine has an advantage, especially in a system
of fluctuating frequency, and it is used to some extent for such
purposes.
This feature of exact step relation is shared also, though to
a lesser extent, by the synchronous motor with self-excitation
by lagging currents, and ordinarily small synchronous motors,
but without field excitation (or with great underexcitation or
overexcitation) are often used for the same purpose.
Machines having more or less the characteristics of the reac-
tion machine have been used to a considerable extent in the
very early days, for generating constant alternating current for
series arc lighting by Jablochkoff candles, in the 70's and early
80's.
Structurally, the reaction machine is,similar to the inductor
machine, but the essential difference is, that the former operates
by making and breaking the magnetic circuit, that is, periodically
changing the magnetic flux, while the inductor machine operates
by commutating the magnetic flux, that is, periodically changing
the flux path, but without varying the total value of the magnetic