Ohmmeter

An ohmmeter is an instrument for measuring electrical resistance.

Its design consists of a small battery to apply a voltage to the low-measure resistance, and then, using a galvanometer, measure the current that flows through the resistance.

The scale of the galvanometer that is calibrated directly in ohms, since in application of Ohm's law, since the battery voltage is fixed, the current flowing through the galvanometer will only depend on the low resistance value measurement, that is, the lower the resistance, the higher the current intensity and vice versa.

There are also other types of ohmmeters that are more accurate and sophisticated, in which the battery has been replaced by a circuit that generates a current of constant intensity I, which is circulated through the resistance R under test. Then, using another circuit, the voltage V is measured at the ends of the resistance. According to Ohm's law, the value of R will be given by:

R=VI{displaystyle R={frac {V}{I}}}}

For high-precision measurements, the arrangement indicated above is not appropriate, since the meter reading is the sum of the resistance of the measurement leads and that of the resistance under test.
To avoid this drawback, a precision ohmmeter has four terminals, called Kelvin contacts. Two terminals carry the constant current from the meter to the resistance, while the other two allow the measurement of the voltage directly between its terminals, with which the voltage drop in the conductors that apply said constant current to the resistance under test does not affect the accuracy of the measurement.

The ohmmeter was invented by the German physicist George Simon Alfred Ohm.

Types of Ohmmeters

• Wheatstone Bridge
• Voltmeter
• Fine
• Galvanometer