# How do traffic signal induction loops work?

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And what is induction, anyway?

In: Engineering

When a wire experiences changing magnetic pull, voltage is generated in that wire. This is called induction.

When electricity runs through a wire, it becomes a magnet, temporarily.

So when you try to pass electricity through a wire, especially a coiled one where each loop magnetically interacts with the other loops because they’re right next to eachother, it’s kind of difficult. The electricity takes time to build up in the loop, as the voltage from the magnetism fights against the voltage you’re using to push electricity through the wire. Then, when you stop applying a voltage, it takes a while for the electricity to stop as the voltage from the magnetism is now keeping it going

Another way to put this: Inductance is a ‘force’ that fights to keep the same amount of electricity running through a wire. If you want more or less electricity to flow, you need to use a voltage. A coil of wire has more inductance than a single wire, and a coil of the same size with more loops has more inductance.

You can measure inductance pretty easily. You try to pass electricity through a wire and see how long it takes to get going.

If you bring a magnetic material like iron near the loop, it amplifies the magnetic strength of the wire and so increases the inductance. If you bring a non-magnetic conductive material like copper near the loop, it allows electricity to freely flow which reduces the inductance.

Traffic signals put a big loop of wire in the ground and repeatedly test its inductance, and when this inductance changes enough they know that a large metal object is nearby.

Ever made an electromagnet? You can wrap wire around an iron nail and connect a battery to it. Current passing through the coil of wire induces a magnetic field in the nail. It even works in reverse- if you pass a coil of wire through a magnetic field a voltage will be induced in the wire. Electric guitar pickups consist of a coil of wire around a permanent magnet. The vibrating string distorts the magnetic field which induces the signal that the amplifier picks up. Metal detectors work by pulsing current through a coil. The brief pulse of current induces a magnetic field around the coil, which quickly collapses as the pulse ends. As the field collapses it induces its own signal in the wire which can be read by onboard processing equipment. The presence of materials which interact with a magnetic field (typically metals) slightly alters the shape of the collapsing magnetic field which tells the metal detector there’s metal in range. A traffic signal induction loop is essentially a massive metal detector embedded in the ground pointing upwards.