Various refinements make a good thermostat reliable and noiseless. It should have a magnetic “make,” which means that the strip, once it gets near to completing the circuit, is snapped over into position by a small magnet, thus preventing sparking or arcing. The contacts should be of silver to prevent excessive wear, and there should be a condenser or capacitor to prevent radio interference. If it is not built into the instrument, it may be added afterwards quite easily; a suitable condenser is about 0.02 microfarad.
The thermostat is nearly always non-submersible and is clipped onto the side of the tank. It has a glass body, with the works inside it, and a control, which consists usually of a small adjustable screw with a non-conducting portion so that the operator can alter the setting of the bimetallic strip, and hence the temperature of the tank. Often a pilot light is present which indicates when the thermostat is operating.
There are more expensive models in which the thermostat proper is outside the tank, and a temperature “feeler” is placed in the tank like a submersible heater. Some combination models of thermostat- lus- heater are also available and seem to be satisfactory. However, they can be used only in a single tank and hence each combination must be repeated for every additional tank.
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