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.

 

You should not hijack another member's posting. Even if the subject is closely related, start your own thread.