The movement of the sliding-contact _q_, fig. 19, along the slide-wire
J, is produced by means of a special device called a creeper, consisting
of a piece of brass carefully fitted to a threaded steel rod some 30
centimeters long. The movement of this bar along this threaded rod
accomplishes two things. The bar is in contact with the slide-wire J
and therefore varies the position of the point _q_ and it also carries
with it a stylographic pen. The movements of this bar to the right or
the left are produced by an auxiliary electric current, the contact of
which is made by a plunger-plate forcing the pointer P against either
S_{1} or S_{2}. P makes the contact between Pl and either S_{1} or S_{2}
and sends a current through solenoids at either the right or the left of
the creeper. At intervals of every 2 seconds the plunger rises and
forces the pointer P against either S_{1}, _i_, or S_{2} above. The
movement of this plunger is controlled by a current from a 110-volt
circuit, the connections of which are shown in fig. 22. If the contact
is made at T, the current passes through 2,600 ohms, directly across the
110-volt circuit, and consequently there is no effective current flowing
through the plunger Pl. When the contact T is open, the current flows
through the plunger in series with 2,600 ohms resistance. T is opened
automatically at intervals of 2 seconds by the clock.
[Illustration: FIG. 21.--Diagram of wiring of circuits actuating plunger
and creeper.]
[Illustration: FIG. 22.--Diagram of wiring of complete 110-volt
circuit.]
The movement of the contact arm along the threaded rod is produced by
the action of either one of two solenoids, each of which has a core
attached to a rack and pinion at either end of the rod. If the current
is passed through the contact S_{1}, a current passes through the
left-hand solenoid, the core moves down, the rack on the core moves the
pinion on the rod through a definite fraction of a complete revolution
and this movement forces the creeper in one direction. Conversely, the
passing of the current through the solenoid at the other end of the
threaded rod moves the creeper in the other direction. The distance
which the iron rack on the end of the core is moved is determined
carefully, so that the threaded rod is turned for each contact exactly
the same fraction of a revolution. For actuating these solenoids, the
110-volt circuit is again used. The wire connections are shown in part
in fig. 21, in which it is seen that the current passes through the
plunger-contact and through the pointer P to the silver plate S_{1} and
then along the line G_{1} through 350 ohms wound about the left-hand
solenoid back through a 600-ohm resistance to the main line. The use of
the 110-volt current under such circumstances would normally produce a
notable sparking effect on the pointer P, and to reduce this to a
minimum there is a high resistance, amounting to 10,000 ohms on each
side, shunted between the main line and the creeper connections. This
shunt is shown in diagram in fig. 22. Thus there is never a complete
open circuit and sparking is prevented.