The calibration characteristic of ohmmeter is different from that of ammeter and voltmeter. Ohmmeter has the following remarkable characteristics:
(1) the more rightward the scale of the ammeter and the voltmeter is, the larger the ohmmeter is. This is because the smaller rx is, the larger i is. When rx is equal to infinity, and i is equal to 0, it's at the far left; when rx=0 (short connection of two meter pens), i is ig, and the resistance at full scale of the ammeter is "0" At the far right.
(2) uniform calibration of ammeter and voltmeter. The scale is very uneven, the more to the left, the more dense. This is because e, r, and rx are all constant after the zero point correction, and i varies with rx. But they are not simple linear scaling relationships. So the dial is not uniform.
(3) the scale of ammeter and voltmeter is from 0 to a certain value, so each meter has a certain range.
(4) both the ammeter and the voltmeter require a circuit connection (that is, series or parallel) when they are in use, but the ohmmeter can read the number directly without connecting the circuit.