To measure current with an ammeter it's nearly always wired in series - so you need to interrupt an existing wire (cut it) and join the two ends to the meter. If you wire the Amp meter any other way the indication will be meaningless at best - fuse blowing or wires melting at worst
An ammeter with a moving coil usually has a low resistance "shunt" built into it - Most of the current passes through the shunt but a small proportion passes through the moving coil giving you the reading.
A moving coil instrument takes only a very small amount of current before it destroys itself - that's why they put a shunt inside to take the bulk of the Amps. The moving coil actually measures the voltage drop (mV) across the shunt.
See...
http://en.wikipedia.org/wiki/Ammeter
Peccavi, October 2009
