US Patent 7170424 Oil well casting electrical power pick-off points

A power supply apparatus is provided for supplying power and communications within a first piping structure. An external power transfer device is positioned around the first piping structure and is magnetically coupled to an internal power transfer device. The internal power transfer device is positioned around a second piping structure disposed within the first piping structure. A main surface current flowing on the first piping structure induces a first surface current within the external power transfer device. The first surface current causes a second surface current to be induced within the internal power transfer device.