and meter as shown in Figure 3. The meter will have two
connection posts at the back with the positive marked
with a plus sign (+). Be sure to observe the polarity of the
power supply and meter when making the connections.
Failure to do this can result in damage to the meter.
3. Set the power supply voltage to about + 5 volts.
4. Slowly decrease the amount of resistance in the
decade box until the meter deflects exactly full-scale.
Exceeding the full-scale reading may also damage the
meter. Be careful not to decrease the resistance to less
than 100 ohms.
5. Measure the voltage across the decade box as
accurately as possible.
6. Measure the voltage across the meter movement.
7. Disconnect the power supply from the meter
8. Measure or record the
resistance across the decade box or
If the measured voltage across the
meter is too low to obtain an accurate
reading, then increase the power
supply voltage to 20 volts while
keeping the series resistance of the
decade box to no less than 400 ohms
to prevent damage.
If full meter deflection has been
achieved, then all the information
necessary to determine the full-scale
current of the movement and to also
calculate the meter internal resistance
is now available.
If a DMM is unavailable to make
an accurate voltage measurement, then inexpensive high-input impedance digital panel meters with millivolt
accuracy are available on the Internet for about $5 as
shown in Figure 4.
Circuit current (I) = Resistor voltage (E) / Resistor
Meter full-scale current = Circuit current.
Meter internal resistance = Voltage across the meter /
There are two problems with the above approach
when testing an unknown meter:
1. The meter may have a built-in shunt across the
movement internally in the case of a direct-reading
ammeter. This is quite common in ammeters up to 10
2. The meter may have a built-in internal series resistor
in the case of a direct-reading voltmeter. This is quite
common in voltmeters up to a few hundred volts.
In the case of a direct-reading ammeter, the wattage
rating of the decade resistance box could easily be
exceeded. If we assume that the decade box or
potentiometer is rated at 1/4 watt, then (in the worst case)
with five volts applied to the circuit, the resistance should
not be adjusted lower than 100 ohms to prevent damage
to the resistor(s).
If the meter contains an internal shunt, then it may not
fully deflect even with 100 ohms in series. If it is a direct-reading ammeter, then there will be close to 50
milliamperes flowing in the circuit without full deflection.
In the case of a direct-reading voltmeter, the meter
will read very close to the applied power supply voltage
when the resistance is adjusted down to our minimum of
100 ohms, and the voltage across the series resistor will
FIGURE 4. Digital panel meters.
44 October 2017