Power from the battery flows into the steering column, where it enters the turn signal switch. The switch has 3 positions: off, left and right. When a left or right turn is indicated, the electricity flows through a device called the thermal flasher to the front and rear turn indicator bulbs, and to the dashboard turn signal indicator lights. The thermal flasher is the heart of the turn signal. It turns the light on and off using a process that has not changed in decades.
The thermal flasher has a piece of slightly bent spring steel next to an electric contact. The whole thing is surrounded by a coil of resistor wire. Initially, the spring steel is not touching the electrical contact. The light is off, and nearly all of the power goes through the resistor wire, which causes it to heat up. As it heats up, it heats the steel, which causes it to straighten and touch the contact. When it does, the electricity goes through the spring steel into the contact and powers the signal lights. Since little power is now going through the resistor wire it cools down, causing the steel to cool and bend again. The steel bends away from the contact, turning off the power and starting the whole process again.
In recent years, LEDs have become more common in signal lights and headlights on cars. LEDs have a number of advantages over incandescent bulbs. They last much longer, use less energy, and stay cool. They are also less vulnerable to water. A small bit of water under the cover of a conventional bulb can easily destroy it. The cool water can damage a very hot bulb by cooling it too rapidly. LEDs never get hot, and can't be easily destroyed by water. Another innovation is the addition of turn signals on the side view mirrors. This makes it easier for drivers to the side of a car to know when it is turning or changing lanes.
