The tiny heater that once made turn signals blink
In older cars, the familiar turn-signal click came from a thermal flasher: a small switch that used heat to interrupt and restore electrical current. The sound was not added merely to imitate a signal. It was the mechanism itself moving back and forth.
How heat made the lamps pulse
When the driver switched on a turn signal, current flowed through a narrow strip made by bonding two different metals together. Because the metals expanded by different amounts as they warmed, the strip bent. At a certain point, it pulled away from an electrical contact and broke the circuit, turning the signal lamps off.
With the current cut, the strip cooled and straightened. It touched the contact again, restoring current to the lamps—and starting the heating cycle over. That repeated bend, break, cool, and reconnect produced the flashing rhythm. The switch’s movement made the audible click.
A warning built into the mechanism
This design tied the sound to the operation of the signal. If a lamp burned out, the electrical load could change enough to alter the flasher’s heating cycle. In many vehicles, that meant the signal blinked unusually fast, giving the driver a clue that a bulb needed attention.
Later electronic flashers could control the timing without relying on a heated, bending strip, so the click no longer had to come from the switching mechanism. The old sound began as a by-product of a tiny thermostat-like device: each click marked the moment heat had physically opened or closed the circuit.
