The double finger, T, covers or opens a
small hole in the face, U, communicating by the pipe, W, to the
diaphragm, L. The action of the magnet yoke is to attract the needle
toward the poles of the magnet, while by turning the head the spiral
spring, X, is brought into tension to resist and balance this force, and
can be set and adjusted to any degree of tension. The double finger, T,
turns with the needle, and, by more or less covering the small air inlet
hole, U, it regulates the access of air to the regulating diaphragm, L.
The second finger is for safety in case the brushes get thrown off, or
the magnet circuit be broken, in which case the machine would otherwise
gain a considerable increase of speed before the diaphragm would act. In
these cases, however, the needle ceases to be attracted, falls back, and
the safety finger closes the air inlet hole.
There is no resistance to the free movement of this regulator. A
fraction of a volt increase or decrease of potential produces a
considerable movement of the finger, sufficient to govern the steam
pressure, and in ordinary work it is found possible to maintain the
potential within one volt of the standard at all loads within the
capacity of the machine, excepting only a slight momentary variation
when a large portion of the load is switched on or off.
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