Fig 1Current and resistanceDrag to turn · pinch or scroll to zoomammeter 0.00 A · voltmeter 0.00 V · 1 cell · key out
Ammeter and voltmeter read to their least countBattery, key, rheostat, metersResistance board with four wiresWith the plug in, the ammeter in series reads the current I through the wire and the voltmeter across X and Y reads the potential difference V. More cells, or less of the rheostat, give a bigger current, and V rises in step: V/I stays about 4.4 Ω, so the V–I graph is a straight line through zero and its slope is the resistance (Ohm's law, at a steady temperature). Twice the length doubles R, about twice the area about halves it, and manganin and copper differ from nichrome: R = ρl/A. Inside, free electrons drift slowly against the field while they dart about.