Inductive coupling uses magnetic fields that are a natural part of current's movement through wire. Any time electrical current moves through a wire, it creates a circular magnetic field around the wire. Bending the wire into a coil amplifies the magnetic field. The more loops the coil makes, the bigger the field will be.
If you place a second coil of wire in the magnetic field you've created, the field can induce a current in the wire. This is essentially how a transformer works, and it's how an electric toothbrush recharges. It takes three basic steps:
- Current from the wall outlet flows through a coil inside the charger, creating a magnetic field. In a transformer, this coil is called the primary winding.
- When you place your toothbrush in the charger, the magnetic field induces a current in another coil, or secondary winding, which connects to the battery.
- This current recharges the battery.
You can use the same principle to recharge several devices at once. For example, the Splashpower recharging mat and Edison Electric's Powerdesk both use coils to create a magnetic field. Electronic devices use corresponding built-in or plug-in receivers to recharge while resting on the mat. These receivers contain compatible coils and the circuitry necessary to deliver electricity to devices' batteries.
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A newer theory uses a similar setup to transmit electricity over longer distances. We'll look at how it works in the next section.