Page 71 - Teach Yourself Electricity and Electronics

Ohmmeters 51

3-6 Circuit for using a
microammeter to
measure voltage.

Another type of voltmeter uses the effect of electrostatic deflection, rather than
electromagnetic deflection. You remember that electric fields produce forces, just as do
magnetic fields. Therefore, a pair of plates will attract or repel each other if they are
charged. The electrostatic voltmeter makes use of this effect, taking advantage of the
attractive force between two plates having opposite electric charge, or having a large
potential 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 only
thing 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.

Ohmmeters

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