Figs. 6 and 7 will explain the structure of the commutator. Here a a a
are the segments or contact pieces insulated from each other, and b' b
b are the collecting brushes carried on a spindle, c c'. One of these
brushes, b', is connected to the spindle, c, through an electrical
resistance of plumbago, arranged as shown in Fig. 7, where d e are metal
cylinders, d being in contact with the brush, b', while e is in contact
with the spindle, c. The space, f, between these two cylinders, d e, is
filled with a mixture of plumbago and lampblack of suitable resistance,
confined at the ends by ivory disks. The brush, b', is adjusted by
bending till it remains in contact with any segment of the commutator
for a short time after the other brushes have left contact with that
segment, and thus instead of sudden break of circuit and consequent
sparking, a resistance is introduced, and contact is not broken until
the current has been considerably reduced.

The contact segments are supported at both ends by solid insulating
disks; but they are insulated from each other by the air spaces between
them, where the brushes rub upon them.

The alternating current dynamo of Drs. Hopkinson and Muirhead differs
little in general construction from that we have described; except that
the commutator is very much simplified, and the armature bobbins are
placed opposite each other on both sides of the rim. Instead of forming
the coils into complete bobbins, Dr. Muirhead prefers to wind them in a
zigzag form round the grooved iron rim after the manner shown in Fig. 8,
which represents a plan and section of the alternating current armature.
This arrangement is simpler in construction than the bobbin winding, and
is less liable to generate self-induction current in the armature. Sir
William Thomson has adopted a similar plan in one of his dynamos. In
Fig. 8, a is the pulley fixed to the spindle of the machine, b b is