# Hands-On-PhysicsHEAT & TEMPERATURE

## - Thermostat -

Figure C7
Thermostat

A thermostat is just a switch which turns on and off as temperature changes. When a switch completes an electrical connection it is "on" and when it breaks the electrical connection it is "off". The thermometer you'll use for the thermostat uses a bimetallic spiral to turn a dial. It is constructed of aluminum which conducts electricity and the dial is used for the moving part of the switch. This temperature switch turns on when the motion of the dial makes it touch a stationary contact to complete an electrical connection. If you are using a new thermometer, take off the frame and glass. On the back of this squarish thermometer, there's a small cylinder which holds the bimetallic spiral that moves the dial.

There are three basic problems that must be solved to make this thermometer into a thermostat.

1. The aluminum of the dial does not work well as an electrical contact, copper is better.
2. Another (stationary) contact is necessary for an electrical connection.
3. The thermostat should be adjustable, so the incubator can be set to work at different temperatures.
One solution to problem (a) is to wrap the end of the dial with fine copper wire. One piece of wire from light stranded wire works well. Solid hook-up wire is too heavy. Scrape the the dial near the end to assure clean metal and a good electrical connection. Wrapping the fine wire is a simple but delicate procedure. Another approach is to use solid hook-up wire, wrap one end around the dial and have the other end of this wire touch the contacts (instead of the dial).

Problem (c) can be solved by letting the thermometer pivot in the incubator container. You could make a pivot ring 4 cm in diameter with a hole in the center big enough for the thermometer's small cylinder to stick through. Glue this ring onto the back of the thermometer. This 4 cm pivot ring will fit in 4 cm hole cut in the end of the incubator. When the thermometer and its pivot ring are in place in the end of the incubator, a 6 cm ring can be glued on to the pivot ring to hold everything in place.

Figure C8b
Thermometer Pivot

The problem of the stationary electrical contact(s) may be solved with a couple of bare copper wires held by a "bridge" built over the thermometer. Strip about half a cm of insulation from two pieces of hook-up wire about 10 cm long. Bend the bare wires so they can stick through the bridge to make contact with the thermometer dial. The bridge must hold the bare wires stationary over the turning dial of the thermometer. It should be attached to the end of the incubator about a millimeter above the aluminum thermometer, so the thermometer can rotate freely. Look at figure C4, at the top of this page,which shows the completed thermostat with the bridge covering the bottom part of the thermometer. Cut out a bridge and check to be sure it fits the end of the incubator. Glue down the wires.

Figure C9a
Thermostat Bridge

The pieces you have made will go together at the end of the incubator to become a thermostat for regulating the air temperature in the chamber.

Figure C9b
Thermostat Pieces

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