The phone would emit magnetic waves and the tattoo would act as a receiver.

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GoalRef uses a microchip implanted in the ball and low magnetic waves around the goal.

CNN: Over the line: Goal tech to be used at 2014 World Cup

The phone would communicate with the tattoo through magnetic waves.

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That conundrum was solved by getting the high-frequency signals to penetrate deeper using alternating waves of electric and magnetic fields.

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MRI, which stands for Magnetic Resonance Imaging, is a radiology technique which uses magnetism, radio waves, and a computer to produce a range of images of the body.

BBC: MRI scanner