The engine drives the pump rotor in the compressor directly with a belt and so the compressor needs to be able to be turned on and off on demand. To achieve this, automotive air conditioners use an electromagnetic clutch mechanism to engage the compressor to activate the A/C unit in the cabin. The A/C clutch has a friction plate that is adjacent to an electromagnetic coil. When a current runs through this coil it creates a powerful magnetic force towards the friction plate in the clutch. The compressor's rotor has another friction plate attached to it next to the first one and the electromagnetic force draws it towards the friction plate in the clutch which, when engaged drives the pump rotor in the compressor. The friction plates use springs so that they will naturally return to their disengaged positions when the current stops passing through the magnetic coil to switch off the air conditioner.