Athermostat is able to control the heating systems in a home by using which of the following properties of metals?
1. thermal expansion and contraction
2. freely moving electrons
3. high densities
4. high melting points

option 1. thermal expansion and contraction.

The thermostat uses the thermal expansion and contraction of metals: when the metal expands it acts as a switch, a coiled ribbon expands and the electric circuit is off, then the current stops. When the temperatures decrease, the coiled ribbon contracts and the circuit is on, then the current starts to flow and the resistance starts to radiate heat.

The duration of a photographic flash is related to an rc time constantly