The starting vibrator is designed to assist the aircraft magneto in starting the engine. Below approximately 150 RPM the magneto cannot produce enough AC voltage to provide a reliable spark. During starting, when the engine is cranking at low speed, the magneto needs help.
The starting vibrator is powered by the aircraft battery and wired to operate only when the starter motor is engaged. When the starter switch is released, the power to the vibrator is disconnected. Using a set of contact points and an electromagnetic coil, the starting vibrator changes the steady DC voltage from the battery into a series of DC pulses. The pulses operate in the magneto like the AC and allows the magneto to produce a hot spark when starting the aircraft engine.
The name starting vibrator comes from the buzzing vibration produced by the rapid opening and closing of the contacts during operation.