Chapter Ohmmeters

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • Another type of voltmeter uses the effect of electrostatic deflection, rather thanelectromagnetic deflection. You remember that electric fields produce forces, just as domagnetic fields. Therefore, a pair of plates will attract or repel each other if they arecharged. The electrostatic voltmeter makes use of this effect, taking advantage of theattractive force between two plates having opposite electric charge, or having a largepotential difference. Figure 3-7 is a simplified drawing of the mechanics of an electro-static voltmeter.The electrostatic meter draws almost no current from the power supply. The onlything between the plates is air, and air is a nearly perfect insulator. The electrostatic me-ter will indicate ac as well as dc. The construction tends to be rather delicate, however,and mechanical vibration influences the reading.OhmmetersYou 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. Thelarger the value of the resistance in series with the meter, the more voltage is needed toproduce a reading of full scale. This has a converse, or a “flip side”: Given a constantvoltage, the current through the meter will vary if the resistance varies. This provides ameans for measuring resistances.An ohmmeter is almost always constructed by means of a milliammeter ormicroammeter in series with a set of fixed, switchable resistances and a battery thatprovides 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 onthe meter is assigned the value of infinity ohms, meaning a perfect insulator. Thefull-scale value is set at a certain minimum, such as 1 Ω, 100 Ω, or 10 KΩ (10,000Ω).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 thebattery be exactly the right kind, and that it be reasonably fresh so that it will provideOhmmeters513-6Circuit for using amicroammeter tomeasure voltage.