Minimum overshoot, small cycling differential and faster response are the most important requirements for control. Based on the differential expansion of metals principle, an increase in temperature causes the brass bottom strip of the Surface Mounting thermostat expand at a greater rate than the internal bridge assembly. Because the base is in direct contact with the heated surface, a temperature change is sensed rapidly.

Mounting 

The heat sensitive base plate of the thermostat must be mounted in full area contact with the heated surface. The surface should be flat and smooth, and screws should be used for mounting. If clamps or brackets are used, they should be carefully designed to assure positive contact and positioning. To bring the control shaft out horizontally, in such applications as waffle irons and sandwich grills, the thermostat may be mounted on an "L" bracket of aluminum with little loss of sensitivity.