Cut from some 1/32-in. sheet brass a piece whose dimensions correspond to those given in Fig. 10.  Drill a 1/8-in. hole, A, in this piece, and cut a slot, B, from one side of the piece into this hole, and a second slot, C, along the center of the piece as indicated in the figure.  Considerable care should be exercised in cutting the slot C, so that its breadth is exactly equal to the diameter of the piece of steel wire fastened on the end of the couple.  Also make 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 be used.