An ohmmeter is an electrical instrument that measures electrical resistance, the opposition
to an electric current.
Micro-ohmmeters make low resistance measurements.
Megohmmeters measure large values of resistance.
The unit of measurement for resistance is ohms.
The first ohmmeters were based on a type of meter movement known as a 'ratiometer'.
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' instead.
These provided no net rotational force to the movement.
Also, the movement was wound with two coils.
One was connected via a series resistor to the battery supply.
The second was connected to the same battery supply via a second resistor and the resistor
under test.
The indication on the meter was proportional to the ratio of the currents through the two
coils.
This ratio was determined by the magnitude of the resistor under test.
The advantages of this arrangement were twofold.
First, the indication of the resistance was completely independent of the battery voltage
and no zero adjustment was required.
Second, although the resistance scale was non linear, the scale remained correct over
the full deflection range.
By interchanging the two coils a second range was provided.
This scale was reversed compared to the first.
A feature of this type of instrument was that it would continue to indicate a random resistance
value once the test leads were disconnected.
Ohmmeters of this type only ever measured resistance as they could not easily be incorporated
into a multimeter design.
Insulation testers that relied on a hand cranked generator operated on the same principle.
This ensured that the indication was wholly independent of the voltage actually produced.
Subsequent designs of ohmmeter provided a small battery to apply a voltage to a resistance
via a galvanometer to measure the current through the resistance.
The scale of the galvanometer was marked in ohms, because the fixed voltage from the battery
assured that as resistance is decreased, the current through the meter would increase.
Ohmmeters form circuits by themselves, therefore they cannot be used within an assembled circuit.
This design is much simpler and cheaper than the former design, and was simple to integrate
into a multimeter design and consequently was by far the most common form of analogue
ohmmeter.
This type of ohmmeter suffers two inherent disadvantages.
First, the meter needs to be zeroed by shorting the measurement points together and performing
an adjustment for zero ohms indication prior to each measurement.
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.
It remains correct at the centre of the scale only, which is why such ohmmeter designs always
quote the accuracy "at centre scale only".
A more accurate type of ohmmeter has an electronic circuit that passes a constant current through
the resistance, and another circuit that measures the voltage across the resistance.
According to the following equation, derived from Ohm's Law, the value of the resistance
is given by:
For high-precision measurements the above types of meter are inadequate.
This is because the meter's reading is the sum of the resistance of the measuring leads,
the contact resistances and the resistance being measured.
To reduce this effect, a precision ohmmeter has four terminals, called Kelvin contacts.
Two terminals carry the current from the meter, while the other two allow the meter to measure
the voltage across the resistor.
With this type of meter, any voltage drop due to the resistance of the first pair of
leads and their contact resistances is ignored by the meter.
This four terminal measurement technique is called Kelvin sensing, after William Thomson,
Lord Kelvin, who invented the Kelvin bridge in 1861 to measure very low resistances.
The Four-terminal sensing method can also be utilized to conduct accurate measurements
of low resistances.
References
See also Megohmmeter
Ammeter Multimeter
Measuring instrument Electronic test equipment
Electronics Electric circuit
List of electronics topics Series and parallel circuits
Galvanometer Rheochord