The ideal step-up transformer, when the primary coil has 1 winding and the secondary coil has 60 windings would step up the voltage 60 times.
With this improvised clamp on AC power meter, which is essentially a transformer, they used 60 windings for the secondary coil, and the live wire is essentially wound once (or half?). I would expect that to step up the voltage 60 times, but instead they only get 0.2 mV - 2 mV AC of output per 1 amp of current of mains flowing through, depending on the core (various cores used were an aluminum carabiner, a zinc/steel key clamp, a key ring). Why is this so low, does the core make such a big difference that what should be a step up of 60 is instead a step down of 100,000? And why is it proportional to the current flowing?
Edit: A couple clarifications: in this setup the primary is getting residential mains voltage, so 120v. A space heater (1kw, so several amps) is turned on, and the transformer is clamped on to one wire (a split extension cord).