e sure to get the sides of this slot perfectly
smooth. Cut from some 1/8-in. brass a disk having a diameter of
1/2 in., and solder it to the end of the needle. The dotted line
in Fig. 10 indicates the proper position of the disk. Now drill a
hole, D, through the disk and needle, of such a diameter that
considerable force must be applied to the steel shaft you have
already made, in order to force it through the hole. Force the
shaft

[Illustration: Shaft for Needle and Needle]

through this hole until the needle is exactly in the center of the
shaft.

The parts of the thermostat thus far made can now be assembled.
Place the steel shaft in its bearings and see that it turns
perfectly free. Then place the steel pin, on the end of the
couple, in the slot C, and fasten the

[Illustration: Wiring Diagram for One and Two Bells]

other end of the couple, by means of two machine screws, to the
support made for the couple. Increase or decrease the temperature
of the thermostat and note the results. If everything is working
all right, the end of the needle should move when the temperature
of the thermostat is changed. The amount the end of the needle
moves can be easily changed by moving the support or the couple
toward or away from the shaft supporting the needle, which changes
the position of the steel pin in the slot C. The nearer the steel
pin is to the shaft supporting the needle, the greater the
movement of the end of the needle due to a given change in
temperature.

A small piece of white cardboard can be mounted directly under the
end of the needle by means of small brass strips, that in turn can
be attached to the lower ends of the main mounting holes D, Fig.
4. A scale can be marked on this piece of cardboard by noting the
position of the needle corresponding to different temperatures as
determined by a thermometer. When this scale has been completed,
you can use the thermostat as a thermometer.

Two contacts may be mounted, one on each side of the needle, in a
manner similar to the method suggested for mounting the cardboard.
These contacts should be so constructed that the end of the needle
will slide over them with little friction, and so that their
position with respect to the end of the needle may be easily
changed.

[Illustration: Assembled Thermostat]

Both contacts must be insulated from the remainder of the
thermostat, and may or may not be connected together, depending on
how the thermostat is to