how to use a current transformer

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: coils-at-dslextreme-dot-com 

Hi List,

Can someone give me a lesson on the use of a current transformer?  I
pulled one out of some junk a few years ago and now I think I can use it.
It is about the size of a big donut, has a 7/8" hole, is 1-1/4" thick, and
has two leads (secondary?), one black and one white, 100:5 printed on it.

I am usiing a variable ballast in my power supply, and I am ready increase
my power level.  I have installed a 30 A fuse on each primary leg of my
transformer, and with my variable ballast I just want to observe the
current.  My handheld meter is only rated for 20 amps.

Can I use a current transformer? Before or after the ballast? Do I read
current or volts at the secondary of the current transformer? Since I only
anticipate 30 amps max, is the ct that I found going to work? Do I pass
one hot line through the hole? Or should I spend money on an amp clamp? If
so, true rms?

I really want to know how to use one of these, but its not necessary.  I
feel pretty comfortable with my calcs. and fuses and circuit breakers are
in place. It will help me to calibrate my homemade inductive ballast
though.
Once that is done, I probably won't need it in the circuit.

Thanks for any insight.

Thad Howard