to insignificant proportions.

[Illustration: HOPKINSON & MUIRHEAD'S DYNAMO-ELECTRIC GENERATOR.]

The general form of this generator is clearly shown by the side and end
elevation.

The armature is made by taking a pulley and encircling it with a rim of
sheet-iron bands, each insulated from the other by asbestos paper. On
one or both sides of the rim thus formed, radial slots are cut to admit
radial coils of insulated copper wire or ribbon, so that they lie in
planes parallel to the plane of the pulley. In the continuous current
machine coils are placed on both sides of the iron rim and arranged
alternately, that on the one side always covering the gap between two on
the other side. In this way, when a coil on one side of the rim is at
its "dead point" and yields its minimum of current, the corresponding
coil on the other side is giving out its maximum.

The field magnets are made in a similar manner to the armature and run
in circles parallel to the rim of the latter. The cores may be built up
of wrought iron as the rim of the armature is; but it is found cheaper
to make them of solid wrought or cast iron. To stop the local induced
currents in the core, however, Messrs. Muirhead and Hopkinson cut
grooves in the faces of the iron cores, and fill them up with sheet-iron
strips insulated from each other, similar to the sheet-iron rim of the
armature.

The coils, both in the armature and electro-magnets, are packed as
closely as they may to each other, and have thus a compressed or
quadrilateral shape. The arrangement is shown in Figs. 1 and 2, which