Transistors Changed Everything. Here’s How They Work

Last week I revealed the sneaky physics behind the “ joule thief ,” an electrical circuit that lets you squeeze more energy out of seemingly dead batteries. The key, it turned out, was a clever pairing of a transformer and a transistor. But the transistor deserves its own headline, because it's maybe only the most important invention of the 20th century.

Life would be completely different without transistors. For starters, there would be no personal computers or cell phones, so no Amazon, no video games, no dating apps, no messaging, no streaming, no social media, no Apple Pay or Google Maps, and no AI (hmm). Cars contain billions of transistors. They're really everywhere. So what is a transistor?

The best way to understand this pivotal technology is to follow the evolutionary trail, all the way back to the electric relays used in telegraphs in the 1800s.

Telegraph lines were basically long electrical circuits. And you probably never thought about it before, but they ran on batteries—primitive, low-voltage batteries that filled an entire closet at the telegraph station—because there was no electrical grid back then.

The electric relay, invented in 1835, was a key component in these systems. Basically, it was a switch that turned an electrical current on and off—sort of like the wall switch that you use to turn a light on. But lacking fingers, the relay used a second electric current to flip the switch. Why would you use a current to turn on a current? That's a good question.

Suppose you wanted to turn on a light in another town 50 miles away. Well, you’d string real long wires on utility poles. Then you could toggle the light on and off, and even use a code with dots and dashes to send text messages. But there was a problem: The longer the wire, the more resistance it had, so not enough current would get through to deliver an intelligible signal.

The solution? Split the circuit into two 25-mile lengths and connect them with a relay. When you closed the switch on the first circuit, it operated the relay, sending the same pattern of current from a second battery through the next circuit and turning on the light. Here's a diagram:

Of course, the telegraph used a buzzer instead of a light, but same idea. Anyway, relays are still widely used today, and the idea of using a current to turn on another current opened up all kinds of possibilities. In cars, for example, it lets you use low-power dashboard controls to switch on high-power circuits that operate things like starters, headlights, or air conditioners.