Electric heater and control box
The electric heating pad is attached to the bottom of the mercury cup by slipping the brass heating pad carrier onto the base of the cup and tightening the small knurled knob.

The picture shows two views of the heating pad carrier. The picture also shows the thermocouple temperature sensing probe that slides into the angled receiver built into the mercury pot front face. The tip of the probe is immersed in the mercury in the cup. The centre portion of the mercury cup has a recess, a 'sump', that allows the probe to be well covrved with mercury without requiring a large total volume in the cup