This is where solid-state ... that it can only control a single circuit, and single throw means that there are only two positions the switch can be in (one on, and one off state).

The pump suction pipe may be earthed and used as one side of the control circuit, the pressure of which (25 v.) is derived from a small double-wound transformer supplied from the power or lighting ...

The IAF scientists claimed to have created an ultra-efficient circuit ... current state of research of less than 90% recharging efficiency for the electrical control of these new heat pumps.