Fenwal Series 30000 Surface Mounting Thermoswitch® controllers operate on the principal of differential expansion of metals. A temperature increase causes the stainless steel outer case to expand at a greater rate than the internal bridge assembly. Because the case is in direct contact with the heated surface, a temperature change is sensed almost instantaneously. With an increase in temperature, the case expands. This results in a linear change of the internal bridge assembly causing the electrical contacts to open. A decrease in temperature closes the contacts. This is a field tried and time tested quality product in the Fenwal tradition.