you're saying that my body's resistance creates an amp load which burns me?
This means that even if there is no amp load downhill from me, if I grab a hot lead my body will then pull an amp load which would vary depending on how much meat and bone it was going through?
In other words, my body would be like a light bulb, consuming energy?
As far as a power line is concerned, yes. If you connect yourself across that line, then current flows through your body at a rate dependent upon your body's resistance (I = E / R). Your body will dissipate heat the same as any other load (P = I x E).
The power line coming into your house has a very low source impedance, comprised of just the internal resistance of the transformer and that of the connecting cables. This source impedance will be just a fraction of an ohm. That's why you can connect a heavy load yet the voltage stays reasonably constant. It's also why you get a massive current flow if you short-circuit the supply (e.g. if the source impedance was 0.1 ohm, then a short-circuit at 120V would result in I = 120 / 0.1 = 1200 amps).
A megger, on the other hand, has a much higher source resistance. A 500V megger which can deliver a maximum of, let's say 5mA, would have a source resistance of 100,000 ohms. It's that high internal resistance which limits the current.
If you touched the probes in such a way that the resistance through your body was also 100,000 ohms, then the 500V source would be divided equally between the megger's internal resistance and that of your body. You would experience a shock of 2.5mA at 250V.
