noids, 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.

THE CLOCK.

The clock requires winding every week and is so geared as to move the
paper forward at a rate of 3 inches per hour. The contact-point for
opening the circuit T on fig. 22 is likewise connected with one of the
smaller wheels of the clock. This contact is made by tripping a little
lever by means of a toothed wheel of phosphor-bronze.

INSTALLATION OF THE APPARATUS.

[Illustration: FIG. 23

Temperature recorder. The recorder with the coordinate paper in the
lower box with a glass door. A curve representing the temperature
difference between the ingoing and outgoing water is directly drawn on
the coordinate paper. Above are three resistance boxes, and the switches
for electrical connections are at the right. On the top shelf is the
galvanometer, and immediately beneath, the plug resistance box for
altering the value of certain shunts.]

[Illustration: FIG. 24.--Detailed wiring diagram showing all parts of
recording apparatus, together with wiring to thermometers com