ohmmeter

A small portable instrument for measuring resistances in an electrical circuit. It consists of an ammeter together with one variable and one fixed resistance. The unknown resistance is found by measuring the difference in current flow between closed circuit conditions and when the unknown resistance is introduced between the connectors of the ohmmeter.

An Ohmmeter is an electrical measuring instrument that measures electrical resistance, the opposition to the flow of an electric current. The scale of the galvanometer was marked in ohms, because the fixed voltage from the battery assured that as resistance decreased, the current through the meter would increase.

A more accurate type of ohmmeter has an electronic circuit that passes a constant current I through the resistance, and another circuit that measures the voltage V across the resistance. According to the following equation, derived from Ohm's Law, the value of the resistance R 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. 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.