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 them well with solder.  Place the two tinned surfaces just treated in contact with each other and heat them 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

[Illustration:  Support for Couple, and Needle-Mounting Strip]

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.

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.