Page 70 - Teach Yourself Electricity and Electronics

50 Measuring devices

3-5 A resistor can be
connected across a meter
to reduce the sensitivity.

voltage placed across it. Early electrical experimenters recognized that an ammeter
could be used to measure voltage, since an ammeter is a form of constant-resistance
circuit.
If you connect an ammeter directly across a source of voltage—a battery, say—the
meter needle will deflect. In fact, a milliammeter needle will probably be “pinned” if you
do this with it, and a microammeter might well be wrecked by the force of the needle
striking the pin at the top of the scale. For this reason, you should never connect mil-
liammeters or microammeters directly across voltage sources. An ammeter, perhaps
with a range of 0-10 A, might not deflect to full scale if it is placed across a battery, but
it’s still a bad idea to do this, because it will rapidly drain the battery. Some batteries,
such as automotive lead-acid cells, can explode under these conditions. This is because
all ammeters have low internal resistance. They are designed that way deliberately.
They are meant to be connected in series with other parts of a circuit, not right across
the power supply.
But if you place a large resistor in series with an ammeter, and then connect the
ammeter across a battery or other type of power supply, you no longer have a short cir-
cuit. The ammeter will give an indication that is directly proportional to the voltage of
the supply. The smaller the full-scale reading of the ammeter, the larger the resistance
to get a meaningful indication on the meter. Using a microammeter and a very large
value of resistor in series, a voltmeter can be devised that will draw only a little current
from the source.
A voltmeter can be made to have different ranges for the full-scale reading, by
switching different values of resistance in series with the microammeter (Fig. 3-6). The
internal resistance of the meter is large because the values of the resistors are large.
The greater the supply voltage, the larger the internal resistance of the meter, because
the necessary series resistance increases as the voltage increases.
It’s always good when a voltmeter has a high internal resistance. The reason for this
is that you don’t want the meter to draw much current from the power source. This cur-
rent should go, as much as possible, towards working whatever circuit is hooked up to
the supply, and not into just getting a reading of the voltage. Also, you might not want,
or need, to have the voltmeter constantly connected in the circuit; you might need the
voltmeter for testing many different circuits. You don’t want the behavior of the circuit
to be affected the instant you connect the voltmeter to the supply. The less current a
voltmeter draws, the less it will affect the behavior of anything that is working from the
power supply.