Hi! You can puchase a ring core from most electronics component suppliers. Look for one that is big enough so that you can easily thread insulated wire through it in the form of a winded coil. Use say, 50 turns for a start. Also, pass a single insulated wire through the hole of the ring. This is the basic construction of a CT (current transformer).
You'll now need a variable ac source (LOW VOLTAGE FOR YOUR OWN SAFETY!). This may be a "VARIAC" with mains input to it and the output connected to a small step-down transformer, say 12V, 2 amps rated.
Put an ac ammeter (0 - 5A) in series with the single wire that you put through the ring and connect the 2 ends to the 12V winding of the transformer. (POWER OFF at this stage!) Connect the free ends of the coil that you've wound to another ac ammeter (0 - 100mA AC).
See that the "variac" is adjusted to minimum position. Switch on AC supply. VERY SLOWLY, GRADUALLY INCREASE THE INPUT VOLTAGE to the step-down transformer. (By turning the knob of the "Variac").
Watch both ac meters carefully. STOP when you reach 1 amp on the first one and observe the reading of the second one. It should read 1/50 of the input current if the core is not saturated (20 mA). Never turn the "Variac" up more, because you'll exceed the step-down transformers capabilities! (Remember, it is effectively shorted out in this experiment!)
First off Great note, very descriptive. I have a wrench to throw however. I am trying to do the same thing, however, I need to measure milli-amp currents from the main at high voltages, in excess of 15KV. Here's my main issue, my primary is adjustable 15Kv 30ma, I need to measure the load current (0 to 30ma). The concern is, at 15kv through a so 50:1 current transformer will the voltage be an order of magnatude higher or am I missing something. I will be experimenting soon, but I also do not want to fry anything in the attempt.
Thanks in advance.