screw. The two magnet
cores P are screwed into the front plate V, which is also of gun-metal, and
after the bobbins R have been slipped on, the shanks of the cores are
passed through holes drilled in the flange N of the main casting and held
in place with nuts. The faces of both A and B must be turned perfectly
square with the shaft, so that they run accurately together. The portion B
is {91} kept in contact with A by means of a spring S, the pressure being
regulated by the collar H. Current is taken to the magnets by means of the
two insulated copper rings D mounted upon the body of A. The gear-wheels on
both portions have teeth of very fine pitch, the number of teeth on each
being regulated by the speed of the driving motor and the required machine
speed. Connection with the circuit breaker L and the battery B^2 is made
with the collecting rings D by the brushes T. The complete connections are
given in the diagram Fig. 51.

_The Isochroniser._--This is a device for ensuring the correct speed
regulation of the driving motors, and is shown in detail in Fig. 48. It
comprises two portions, one portion being rotated at a definite speed by
electrical means, and the other portion rotated by the driving motor.

The main portion consists of a metal tube N, bushed at both ends, the
bottom end of the tube being arranged to work on ball-bearings. An ebonite
bush C carries three copper rings T, T^1, T^2, and the brushes R, R^1, R^2
are in electrical contact with them. The ebonite plate J, 3-1/2 inches
diameter, is secured to the top end of N, and carries a contact piece Q,
shown separate at E. As will be seen this is a block of ebonite with three
contacts arranged on the top surface. The middle contact P is 1/64th of an
inch wide, and the contacts P^1 {92} and P^2 are placed on either side at a
distance of 1/16 inch; the contact strips P^1, P^2 carry the brass pins D,
which are about 1/16 inch diameter, and spaced 3/8 inch apart. A connecting
wire is carried from the contact P to the copper ring T, another from P^1
to T^1, and one from P^2 to T^2.

[Illustration: FIG. 48.

N, brass tube; S, bushes; G, ball-bearing; H, gear-wheel; T, T^1, T^2,
copper rings; C, insulating block; R, R^1, R^2, brushes; J, ebonite disc;
Q, contact block; D, metal pins; O, pulley, P, P^1, P^2, contact plates; K,
needle; Z, spring; W, steel rod; E, countersunk bearing.]

The bushes S are bored a running fit for the steel rod W (shown separate at
A),