Final answer:
The device inside a sequencer that causes the contacts to close is an electromagnet, which creates a magnetic field when voltage is applied, thus causing the movement of the sequencer's armature and closing the contacts.
Step-by-step explanation:
The device inside a sequencer that causes the contacts to close when voltage is applied to the coil is known as an electromagnet. When the sequencer coil receives voltage, the resulting electrical current generates a magnetic field. This magnetic field works to physically move the sequencer's armature or plunger, leading to the contacts closing. The process is reversible, meaning that when the voltage is removed, the magnetic field dissipates, allowing springs or other mechanisms to open the contacts again.
Sequencers are commonly found in HVAC systems to control the operation of heating elements and fans in a sequenced manner, preventing a sudden surge in electrical draw. The electromagnet is a crucial component because it translates an electrical signal into a physical action, thus enabling control over electrical circuits without the need for manual intervention.