How to build a non-contact (term I just made up, but seems accurate)ammeter.
More specifically there is a variety of ammeter that has a ring that
can be opened. After the ring is opened, 1 conductor of an AC circuit
is placed within the ring. The ring is then allowed to close. The
device then gives an accurate measurement of the current electrical
draw on said circuit. Please see an example (cheap but functional) of
this type of device at:

http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=22-602

For a project we are trying to build a functioning (repeatable
measurements) version of the tool. A standard multitester may be used
to interpret reading, but the actual "ring" or whatever it is must be
created by us. It does not need to open as described above. If
possible, I would also appreciate any info on what factors determine
the sensativity of the device.
Thanks in advance

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Clarification of Question by yournicknamehere-ga on 19 Oct 2004 00:59 PDT

I wasn't very clear, but I need this to measure AC only.

Dear yournicknamehere,

In affect what you are looking for is what is known as a current
transformer. There are standard classes for these instruments which
ensure accuracy and repeatability.

for Current transformer design basics see:

http://leeh.ee.tut.fi/transformer/tra351.htm

There is more information available at:

http://www.ecmweb.com/mag/electric_dont_left_short/
IEEE 52, Application Guide for Grounding of Instrument Transformer
Secondary Circuits and Cases.

But please remember that when the primary circuit is "ON", the
secondary measuring circuit should never be kept open! If you propose
to use a multimeter for measuring secondary current, use a shoting
link in this circuit.

With best wishes.

This link could be helpful to you:
http://www.kilowattclassroom.com/Archive/AN0008.pdf

The common name for the device you are looking for is a "current
clamp", also known as a non-contact ammeter.  The physics of these
devices have been described in a couple of journal articles:

D.C. Henry, Enhancing electromagnetism experiments with clamp-on
ammeters. American Journal of Physics, Vol. 69, No. 1, pp. 76?78,
January 2001 (http://scitation.aip.org/journals/doc/AJPIAS-ft/vol_69/iss_1/76_1.html#R7)

P. Heller, American Journal of Physics Volume 60, no. 1, pp. 17-25.
(http://scitation.aip.org/getabs/servlet/GetabsServlet?key=AJPIAS&prog=spinref&id=AJPIAS000060000001000017000001&idtype=cvips&linksmith=yes)

Briefly, these devices work beause the AC current in the wire being
measured creates a varying magnetic field around the wire.  The
magnetic field will, in turn, induce a current in a coil (the loop of
the ammeter), which allows one to infer the current in the first wire.
The accuracy of these devices is not that great (a couple of percent),
and they are typically used for measuring fairly high currents.

A more sophisticated (and more sensitive) version of the clamp ammeter
is based on the Hall Effect -- the phenomenon by which a potential
difference is created perpendicular to the direction of current flow
in a conductor in a magnetic field.  A complete design, including
parts list, for such a device that can be hooked up to a standard
multimeter can be found at:
http://www.siliconchip.com.au/cms/A_30669/article.html

In addition to the excellent comments, easy way is to feed a single
turn of wire round the central core of a small mains transformer --
monitoring with an ammeter on the primary winding. You will
unfortunately get quite a large voltage on the secondary though so it
is best to strip the secondary off first. This is quite easy with
split bobbin designs. Alternatively one could strip the primary and
monitor on the secondary which would give you a much higher reading.
If you can dissemble the laminations you can make a clamp-on device.

Best