Ammeter vs voltmeter

An ammeter measures current, so it must sit in series — the current you want to measure has to physically flow through it. To do that job without changing the very current it’s measuring, an ideal ammeter has near-zero resistance: it acts almost like a plain piece of wire.

A voltmeter measures voltage, so it sits in parallel — across the two points you’re interested in. To avoid stealing any current and skewing the reading, an ideal voltmeter has near-infinite resistance: it acts almost like an open circuit, sipping a negligible trickle.

This is also why a real multimeter has a separate physical socket for the amps probe: it forces you to consciously rewire into series for current, instead of accidentally shorting things by leaving the probe where you measure voltage.