Ameter ?s

So an ameter measures current, specifically the current passing through the ameter, and a volt meter measures the difference between two points?

3 questions:
What is the wire that the ameter is spliced into?
What 2 wires is a volt meter hooked into? (its 2 right, or 3?)
Is the downside to the ameter that you actually have current running through the interior of the car?

I have 3 SW gauges, all match, all old, and I would like to make the ameter work as long as it is safe.