An older type of thermostat used in furnace and heat pump control is shown in ? Fig. 5.2...

An older type of thermostat used in furnace and heat pump control is shown in ? Fig. 5.21. The glass vial tilts back and forth so that electrical contacts are made via the mercury (an electrically conducting liquid metal), and the furnace or heat pump is turned off and on. Explain why the vial tilts back and forth. (Newer thermostats are electronic.)