Ohmmeter

Micro-ohmmeters (microhmmeter or micro ohmmeter) make measurements of low resistance.

Megohmmeters (also a trademarked device Megger) measure large values of resistance.

[1][2] These were similar to the galvanometer type movement encountered in later instruments, but instead of hairsprings to supply a restoring force they used conducting 'ligaments'.

Ohmmeters of this type only ever measured resistance as they could not easily be incorporated into a multimeter design.

The scale of the galvanometer was marked in ohms, because the fixed voltage from the battery assured that as resistance is increased, the current through the meter (and hence deflection) would decrease.

This is because as the battery voltage decreases with age, the series resistance in the meter needs to be reduced to maintain the zero indication at full deflection.

Second, and consequent on the first, the actual deflection for any given resistor under test changes as the internal resistance is altered.

These measurements are then digitized with an analog digital converter (adc) after which a microcontroller or microprocessor make the division of the current and voltage according to Ohm's law and then decode these to a display to offer the user a reading of the resistance value they're measuring at that instant.

To reduce this effect, a precision ohmmeter has four terminals, called Kelvin contacts.

The Four-terminal sensing method can also be utilized to conduct accurate measurements of low resistances.