If equal lengths of two different metals, such as copper and iron, are riveted together so that they cannot move separately, they form a bimetallic strip (Figure 1).
When heated, copper expands more than iron, and to allow this the strip bends with copper on the outside (Figure 2).
If they had expanded equally, the strip would have stayed straight.
Bimetallic strips have many uses.
Uses of Bimetallic strips
Bimetallic strips are used in fire alarms and thermostats.
a) Fire alarm
The heat from the fire makes the bimetallic strip bend and completes the electrical circuit, so ringing the alarm bell (Figure 3).
A thermostat keeps the temperature of a room or an appliance constant. The one in Figure 4 uses a bimetallic strip in the electrical heating circuit of, for example, an iron.
When the iron reaches the required temperature the strip bends down, breaks the circuit at the contacts, and switches off the heater. After cooling a little the strip remakes contact and turns the heater on again. A near-steady temperature results.
If the control knob is screwed down, the strip has to bend more to break the heating circuit and this needs a higher temperature.