If all the heating elements in a furnace were to be switched on at the same time, they would trip the circuit breaker.
The sequencer avoids this by switching the elements in sequence. The sequencer consists of a thermostat. The thermostat senses the furnace Temperature and activates the elements.
The thermostat sends a current to a small element in the sequencer. This elements heats up a bimetallic strip which closes and supplies power to the heater element. When the supply to the heater coil is cut off, the bimetallic strip cools and open the circuit. The heating element is thus switched off.
The sequencer is thus able to switch on and off the heater by means of bimetallic strips.