I've read that the human body has resonant frequencies somewhere around 50MHz. I also think it would be interesting to look for values in other ranges like 10-100 kHz. I'm wondering how to build a device which can measure these frequencies and track them as they change.
I was thinking about using a circuit designed for a crystal oscillator, and just putting a human where the crystal would be. However, I'm not sure about the various ways in which the analogy could fail. Also, usually circuits for crystals specify a very specific oscillation frequency. I might be looking for a circuit where, for instance, I specify a wide range like 20-40 kHz, and if I connect it to some parallel LC circuits with a resonant frequencies of 15 kHz and 34 kHz and 100 kHz, then it will oscillate at 34 kHz because that is where the impedance is minimized within the specified range.