OHMMETER

You remember that the current through a circuit depends on the resistance. This prin- ciple can be used to manufacture a voltmeter using an ammeter and a resistor. The larger the value of the resistance in series with the meter, the more voltage is needed to produce a reading of full scale. This has a converse, or a “flip side”: Given a constant voltage, the current through the meter will vary if the resistance varies. This provides a means for measuring resistances. An ohmmeter is almost always constructed by means of a milliammeter or microammeter in series with a set of fixed, switchable resistances and a battery that provides a known, constant voltage (Fig. 3-8). By selecting the resistances appropri- ately, the meter will give indications in ohms over any desired range. Usually, zero on the meter 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, or 10 KΩ (10,000 Ω). Ohmmeters must be precalibrated at the factory where they are made. A slight er- ror in the values of the series resistors can cause gigantic errors in measured resistance. Therefore, precise tolerances are needed for these resistors. It is also necessary that the battery be exactly the right kind, and that it be reasonably fresh so that it will provide the appropriate voltage. The smallest deviation from the required voltage can cause a big error in the meter indication. The scale of an ohmmeter is nonlinear. That is, the graduations are not the same everywhere. Values tend to be squashed together towards the “infinity” end of the scale.
It can be difficult to interpolate for high values of resistance, unless the right scale is se- lected. 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 until the me- ter is in a part of the scale that is easy to read. Finally, the reading is taken, and is mul- tiplied (or divided) by the appropriate amount as indicated on the range switch. Figure 3-9 shows an ohmmeter reading. The meter itself says 4.7, but the range switch says 1 KΩ. This indicates a resistance of 4.7 KΩ, or 4700 Ω.
Ohmmeters will 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 sub- tracts from, the ohmmeter battery voltage. This in effect changes the battery voltage, and the meter reading is thrown way off. Sometimes the meter might even read “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 need to be sure that there is no voltage between the points under test. The best way to do this is to switch off the equip- ment in question.