which is coned at both ends, and runs between two countersunk bearings,
the bottom bearing E being fixed while {93} the top bearing (not shown) is
adjustable. A needle K is fastened near the end of the rod W, and attached
to this needle is the spring Z, which presses lightly but firmly upon the
contact block Q. To provide a level surface for Z to work over, the spaces
between the contact pieces are filled in with an insulating material, and
the whole surface finished off perfectly smooth. The spring Z is 1/8 inch
wide for portion of its length, but at the point where it presses upon Q it
is reduced in width to 1/64th of an inch (see Fig. 48). The driving
arrangements are as follows. A counter-shaft Q, Fig. 51, fitted with a
grooved pulley, is run in bearings parallel with the shaft W, and is
connected by suitable gearing to the shaft of the driving motor, so that
the needle K makes one revolution in about 2-1/2 seconds. A belt passing
over the pulleys connects the two shafts, and the tension of the belt is
regulated by means of an adjustable jockey pulley.

The tube N, carrying the disc J, must be rotated at a fixed speed, and this
is accomplished in the following manner. An ordinary electric clock impulse
dial, actuated from a master clock, is connected by suitable gearing H, so
that the tube N makes exactly one revolution in 2 seconds; it being
possible to adjust an electric clock of the "Synchronome" type, so that it
only gains or loses about 1 second in 24 hours, and this provides {94} an
accuracy sufficient for all practical purposes. The connections are given
in Fig. 49, and the face of the instrument in Fig. 50. It will be seen that
a connecting wire is run from the steel spindle W to one terminal each of
the lamps L, L^1, L^2, and from the other terminal of the lamps to one
terminal of the batteries J, the battery comprising a set of three 4-volt
accumulators. The other terminals of the batteries are joined one to each
of the brushes R, R^1, R^2.

[Illustration: FIG. 49.]

[Illustration: FIG. 50.

M, terminals for connecting to electric clock; L, white lamp; L^1, blue
lamp; L^2, red lamp.]

The lamps are coloured, the lamp L being white, and the lamps L^1 and L^2
blue and red respectively, and care must be taken in connecting up that
when the needle K makes contact with the stud P the white lamp L is in
circuit. When the machines are working, the operator, by means of the brake
(already described), reduces