If all other factors are held constant, the current through a circuit depends on the resistance. This provides us with a means for measuring resistance. An ohmmeter can be constructed by placing a milliammeter or microammeter in series with a set of fixed, switchable resistances and a battery that provides a known, constant voltage (following image). By selecting the resistances appropriately, the meter gives indications in ohms over any desired range. The zero point on the milliammeter or microammeter is assigned the value of infinity ohms, meaning a perfect insulator. The full-scale value is set at a certain minimum, such as 1 Ω, 100 Ω, 1 kΩ, or 10 kΩ.
A circuit using a milliammeter (mA) to measure dc resistance.
An ohmmeter must be calibrated at the factory where it is made, or in an electronics lab. A slight error in the values of the series resistors can cause gigantic errors in measured resistance. Therefore, precise tolerances are needed for these resistors. That means their values must actually be what the manufacturer claims they are, to within a fraction of 1 percent if possible. It is also necessary that the battery provide exactly the right voltage.
The scale of an ohmmeter is nonlinear. That means the graduations are not of the same width everywhere on the meter scale. The graduations tend to be squashed together toward the infinity end of the scale. Because of this, it is difficult to interpolate for high values of resistance unless the
appropriate meter range is selected.
Engineers and technicians usually connect an ohmmeter in a circuit with the meter set for the highest resistance range first. Then they switch the range down until the meter needle is in a part of the scale that is easy to read. Finally, the reading is taken, and is multiplied (or divided) by the appropriate amount as indicated on the range switch. Following image shows an ohmmeter reading. The meter itself indicates approximately 4.7, but the range switch says 1 kΩ. This indicates a resistance
of about 4.7 kΩ, or 4700 Ω.
An example of an ohmmeter reading. This device shows about 4.7 × 1 kΩ = 4.7 kΩ = 4700 Ω.
Ohmmeters give inaccurate readings if there is a voltage between the points where the meter is connected. This is because such a voltage either adds to, or subtracts from, the ohmmeter’s own battery voltage. Sometimes, in this type of situation, an ohmmeter might tell you that a circuit has “more than infinity” ohms! The needle will hit the pin at the left end of the scale. Therefore, when using an ohmmeter to measure resistance, you must always be sure that there is no voltage between the points under test. The best way to do this is to switch off the equipment in question.