You are now ready to drill the hole A, which should be done as follows: Remove the piece you last mounted and then clamp the main mounting strip in the drill press so that the center-punch mark for the hole A is directly under the point of the drill. Then remount the piece you just removed, without disturbing the piece you clamped in the drill press, and drill a small hole through both pieces. This hole should be about 3/64 in. in diameter. After this small hole has been drilled through both pieces, a countersink should be placed in the drill chuck and the hole in the upper piece countersunk to a depth equal to half the thickness of the metal in which it is drilled. Unclamp the pieces from the drill press, turn over, and countersink the small hole in what was originally the lower piece. The object of countersinking these holes is to reduce the bearing surface of a small shaft that is to be supported in the holes and must be as free from friction as possible.

We may now construct the needle, or moving portion of the thermostat, which should be done as follows: The shaft that is to carry the moving system must be made from a piece of steel rod, about 3/32 in. in diameter. Its dimensions should correspond to those given in Fig. 9. Considerable care should be used in turning this shaft down, to make sure that it fits perfectly in the small holes in the supporting pieces. The shaft should turn freely, but it must not be loose in the holes, nor should it have but a very small end play.

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

Shaft For Needle And Needle

Shaft For Needle And Needle

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 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. Wiring Diagram for One and Two Bells

Illustration: Wiring Diagram for One and Two Bells

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 with little friction, and so that their position with respect to the end of the needle may be easily changed.

Assembled Thermostat

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.

It would be advisable, if possible, to have the part of the needle that touches the contact points, as well as these points, of platinum, as the arc that is likely to be formed will not destroy the platinum as easily as it will the brass. A small wooden containing case can now be made and the thermostat is complete. There should be a large number of holes drilled in the sides, ends and back of the case so that the air inside may be always of the same temperature as the outside air.

In adjusting, testing, or calibrating your thermostat, make sure that it is in the same position that it will be in when in use.

The connections of the thermostat for ringing one bell when the temperature rises or falls to a certain value, are shown in Fig. 11. The connections of the thermostat for ringing one bell when the temperature rises to a certain value and another bell when the temperature falls to a certain value, are shown in Fig. 12. The complete thermostat is shown in Fig. 13.