An electric toothbrush battery can recharge even though there aren't any metal contacts.

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If you own an electric toothbrush, there's a good chance that it has a completely sealed toothbrush unit. The toothbrush rests on the charger without any metal contacts to connect the toothbrush to the base. This has the great advantage of allowing the toothbrush to be completely sealed so that water cannot enter through exposed contacts. It also avoids any problems with water getting into the contacts and shorting out the charger.

This sort of sealed arrangement is called inductive charging. Essentially, the toothbrush and the base form a two-part transformer, with the base having one part of the transformer and the toothbrush having the other. When you slide the toothbrush onto the base, the complete transformer is created and charge can flow.

The base contains one of the coils and the metal bar. The toothbrush contains the second coil. When you drop the toothbrush onto the base, you create the complete transformer!