It is a well known fact, that there is a change in the dimensions of a piece of metal, due to a change in its temperature. This change in dimensions is not the same for all materials; it being much greater in some materials than in others, while in some there is practically no change.

Illustration: Simple Thermostat Couple and Mounting Strip

If two thin, narrow strips of different metals, that contract or expand at different ratio due to a variation in temperature, be rigidly fastened together at their ends, and the combination then heated or cooled, the combined piece will have its shape changed. One of the pieces will increase in length more than the other, due to a rise in temperature, and this same piece will decrease in length more than the other when subjected to a decrease in temperature.

If one end of this combined piece be rigidly clamped to a support, as shown in Fig. 1, and the combination then have its temperature changed, the free end will move to the right or left of its original position, depending upon which of the pieces changes in length the more. If there is a rise in temperature and the right-hand piece B increases in length faster than the left-hand piece A, the free end of the combined piece win move to the left of its original position. If, on the other hand, there is a decrease in temperature, the right-hand piece will decrease in length more than the left-hand piece, and the upper or free end will move to the right of its original position.

Such a combination of two metals constitutes a simple thermostat. If the movement of the free end of the combination be made to actuate a needle moving over a properly calibrated scale. we have a simple form of thermometer. If two electrical contacts, CC, be mounted on the right and left-hand sides of the upper end of the combined piece, as shown in Fig. 1, we have a thermostat that may be used in closing an electrical circuit when the temperature of the room in which it is placed rises or falls a certain value. These contacts should be so arranged that they can be moved toward or away from the combined piece independently. By adjusting the position of these contacts, the electrical circuit will be closed when the temperature of the thermostat has reached an experimentally predetermined value.

The following description is that of a thermostat, constructed by the author of this article, which gave very satisfactory results. First obtain a piece of steel, 6 in. long, 5/8 in. wide and 2/100 in. thick, and a piece of brass, 6 in. long, 5/8 in. wide and 3/100 in. thick. Clean one side of each of these pieces and tin well with solder. Place the two tinned surfaces just treated in contact with each other and heat until the solder on their surfaces melts and then allow them to cool. A better way would be to clamp the two thin pieces between two heavy metal pieces, and then heat the whole to such a temperature that the solder will melt, and then allow it to cool. This last method will give more satisfactory results than would be obtained if no clamps are used, as the thin metal pieces are liable to bend out of shape when they are heated, and as a result they will not be in contact with each other over their entire surfaces. After these pieces have been soldered together forming one piece, which we shall for convenience speak of as the couple, two small holes should be drilled in one end to be used in mounting it, and a notch cut in the other end, as shown in Fig. 2.

Illustration: Support for Couple, and Needle-Mounting Strip

Cut from some thin sheet brass, about 2/100 in. in thickness, two pieces, 1/4 in. wide and 1/2 in. long. Bend these pieces of brass over a piece of hatpin wire, thus forming two V-shaped pieces. Cut off a piece of the hatpin, 5/8 in. long, and fasten it across the notched end of the couple by means of the U-shaped piece of brass, which should be soldered in place as shown in Fig. 3. All superfluous solder should then be cleaned from the couple and the steel pin. Now bend the couple so as to form a perfect half circle, the brass being on the inside.

The base upon which this couple is to be mounted should be made as follows: Obtain a piece of brass, 7 in. long, 3/4 in. wide, and 1/4 in. thick. In this piece drill holes, as indicated in Fig. 4, except A, which will be drilled later. Tap the holes B, C and D for 1/8-in. machine screws.

Cut from some 1/8-in. sheet brass a piece, 1-7/8 in. long and 3/4 in. wide, to be used as a support for the couple. In one end of this piece drill two small holes, as indicated in Fig. 5, and tap for 3/16-in. machine screws. In the opposite end cut a slot, whose dimensions correspond to those given in Fig. 5. Now bend the piece, at the dotted line in Fig. 5, into the form shown in Fig. 6, making sure that the dimension given is correct. This piece can now be mounted upon the piece shown in Fig. 4, by means of two brass machine screws placed in the holes B. The slot in the support for the couple will permit its being moved along the mounting strip, the purpose of which will be shown later.

Next cut another piece of 1/8-in. brass, 2-3/4 in. long and 5/8 in. wide. In this piece drill two 1/8-in. holes, as indicated in Fig. 7, and then bend it at the dotted lines into the form shown in Fig. 8. Mount this strip upon the main mounting strip by means of two brass machine screws placed in the holes C, so that the upper part is over the center-punch mark for the hole A in the main mounting strip.