As the bimetallic strip bends toward and away from contact, an electric arc is created that eventually will pit and corrode the points. To prevent this, a condenser should be attached across the circuit, above the thermostat. Should the condenser short out, the current would bypass the thermostat, which would then not be operating. The heater, with no control on it, would continue to heat.
Better thermostats today have a fuse in series with the condenser. Should the condenser short out, the fuse will blow. The thermostat and heater, however, continue to function without the condenser and fuse assembly until they can be replaced.
Another cause of trouble is water in the thermostat tube. In this situation the heater continues to function but without any shut off control.
Occasionally, although not frequently, the silver points that are used today melt and fuse together. This prevents normal opening of the thermostat and is usually a possibility when the power source is direct current (D.C.). Cleaning the contact points occasionally with very fine sand paper will help correct this trouble.
The most common causes of heaters failing to heat is usually current failure, or improper contacts in the wall plugs. Occasionally the thermostat plug becomes disconnected without anyone realizing it. All that is necessary to restore its function, of course, is to plug it back in.Even more infrequent is a burned out heating oil, or a broken connection within the circuit itself.
Heaters fail to heat only a fraction of the number of times they overheat. It is hoped that the number of times the latter occurs will be lessened considerably by the use of proper fuses.
A magnetic snap action thermostat is a thermostat that operates on the same principle as the ordinary thermostat, but with the following addition. There is a small magnet at the end of the bimetal strip and a corresponding plate on the mounting. As the bending bimetal strip comes close, the magnetic attraction asserts itself, closing the contacts sharply with a snap—hence the name. This rapid closing eliminates the problem of the arc and the necessity for the installation of a radio condenser.