Try to find out where the caps lock is, and unlock it. At the moment you like more like Prostheticus.
For future reference, if you don't know the answer to a question, it is not helpful to tell people that it is in some unspecified technical book somewhere.
If you can identify a specific book that has a specific reference to the problem - with the ISBN for the book and the page or chapter reference for the helpful bit - you can earn brownie points without providing a direct answer.
Unhelpful abuse counts as a waste of bandwidth.
Raise you game or expect to be plonked. But don't worry if Jim Thompson plonks you - he plonks everybody who disagrees with him, which is probably one of the reasons he believes so many things that don't happn to be true.
You seem to have a preconceived notion of what constitutes large, small and insignificant currents levels in terms of the fields they generate, but such categorisations are only relative. "2 or
3 amps" is quite huge in some contexts and generate an appreciable flux in the magnetic core of the clamp. The alternating magnetic field induces a voltage in the clamp's pickup coil and this voltage can certainly reach "a few hundred mV" if enough number of turns are used.
You can also think of the clamp as a current transformer. The wire being measured for current is the primary and the pickup coil of the DMM is the secondary.
If you're more familiar with voltage transformers, think of it this way: Suppose you have just 1 mV output from a microphone. Connect it to the primary of a 1:10 transformer and you will get 10 mV at the secondary terminals. Use a 1:100 transformer and you get 100 mV and so on, theoretically up to any voltage.
At some level, if you wrap a transformer around a wire, you can extract as much or as little power as you like. Consider that, say, 100mV (generated by a 1A flow in the one turn "primary" of your current probe) fed into the 10k impedance of a multimeter is all of 1 *micro*watt, which is pretty much "nothing" in comparison to what the primary is likely to be carrying (e.g., even 1A at 1V is a watt, a million times higher).
The power is coming from the primary, of course: The load on the secondary is reflected back to the primary -- multiplied by the turns ratios of the transformer squared and all. (This load effectively appear in series with thatever the real load on the primary is.) The trick then, is finding sensitive enough meters that the burden on the primary is minimized. You might be surprised at how sensitive some of the old analog meters (galvanometers) are -- 1mA full-scale deflection is what you find in the cheapest instruments, 100uA is found in many mid-grade instruments, and 10uA (and even less) is found in high-end gear.
Wrapping some turns around the power company's lines will get you many, many watts. :-)
I don't think that's quite what John meant. Anyway, that reminds me of a practice by some villagers in my area. They cannot afford, or don't want to pay, the power connection charge and monthly bills. They wire their homes for a few incandescent bulbs and keep a pair of solid-cored wires with the ends stripped bare and bent into a U shape, the other two ends feeding the house wiring. When it gets dark, they use a dry bamboo pole to hook the bare ends to the overhead power lines. Free power - until they get caught. The power company - the government here - usually does nothing more than reprimand the offenders, but the practice is rare now.