dovetailed and interlocked between conical steel rings. The whole is
insulated with asbestos, and, when screwed up by the end nut, forms,
with the steel bush, a compact whole. There are fifteen sections in the
commutator, and each coupling is connected to a section. The whole
armature is bound externally from end to end with brass or pianoforte
steel wire. The magnets are of soft cast iron and of the horseshoe type.
They are shunt-wound only.

On the top of the magnet yoke is the electrical control governor, Fig.
4. It consists of one moving spindle on which are keyed a small soft
iron bar, and also a double finger, T. There is also a spiral spring, X,
attached at one end to the spindle, and at the other to an adjustable
top head and clamping nut, Y. 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