




Meter Resistance


The dc resistance of a milliameter or


voltmeter movement may be determined as
follows:



1.

Connect the meter in series with a
suitable battery and variable resist
ance R_{1} as shown in the diagram above.


2.

Vary R_{1} until a full scale reading is
obtained.


3.

Connect another variable resistor R_{1}
across the meter and vary its value
until a half scale reading is obtained.


4.

Disconnect R_{2} from the circuit and
measure its dc resistance.


The meter resistance R_{M} is equal to the


measured resistance of R_{2}.


Caution: Be sure that R_{1} has sufficient


resistance to prevent an off scale reading
of the meter. The correct value depends
upon the sensitivity of meter, and voltage
of the battery. The following formula can
be used if the full scale current of the meter
is known:


R_{1}

=

voltage of the battery used



full scale current of meter in amperes



For safe results, use twice the value com
puted. Also, never attempt to measure the
resistance of a meter with an ohmeter. To
do so would in all proability result in a
burnedout or severly damaged meter,
since the current required for the operation
of some ohmeters and bridges is far in
excess of the full scale current required by
the movement of the average meter you
may be checking.








Ohms per Volt Rating of a Voltmeter



Where:


=

ohms per volt,


I_{fs}

=

full scale current in amperes.





R

=

shunt value in ohms,


N

=

the new full scale reading divided


by the original full scale reading,
both being stated in the same units,


R_{M}

=

meter resistance in ohms


MultiRange Shunts




R_{1}

=

intermediate or tapped shunt value


in ohms,


R_{1+2}

=

total resistance required for the low


est scale reading wanted,


R_{M}

=

meter resistance in ohms,


N

=

the new full scale reading divided


by the original full scale reading,
both being stated in the same units,

