If Alt-E will allow it Jon S., and while not meaning to "out do" anybody, here is a link to a more, illustrative description of what Jon C. wrote.
http://www.jamestowndistributors.com/userportal/document.do?docId=126&title=Sizing+a+Shunt+to+a+DC+AmmeterIf Alt-E will not, then...
Its funny sometimes what can clear up an understanding of how something works. Take batteries for example. Once I had read from a high school chemistry text book that, a battery doesn't store electricity, that it converts electricity into a chemical energy and stores that, battery types and their ratings were no longer a mystery to me.
Think about the starter motor on an automobile for a moment and how, most of them can "pull" around 80 amps at 12 volts from the cranking battery. Imagine what size of a switch would be on the steering column (picture Frankenstein's laboratory) if there wasn't an electromagnetic contactor or "solenoid switch" on that circuit.
Instead, a small amount of amps runs along a small wire to a small switch on the steering column and then to the solenoid which charges an electromagnet which in turn pulls in the large contact that carries the higher amperage through larger wire to the starter from the battery.
If you think about how an analog ampmeter, with the moving needle, works then you can see that its a tiny little motor of sorts that drives the needle. Now, imagine how big an ampmeter would have to be to read 180 amps of current on 0000 wire.
Picture, David and Goliath.
The shunt (David) knocks down the high amperages (Goliath) and cuts off his head so that the meter (Israeli army) can... Well, maybe thats not such great analogy. But, do you get the picture?