According to the figure, “Common” jack is normally ground
node or reference
terminal of the measured device. The jacks for current measurement and voltage measurement are different.
(1) Voltage measurement
For ideal volt meter,
the impedance of the meter must be as large as possible to
prevent current loss which leads to measurement error. In the figure, a resistor is connected to the galvanometer in series to increase the meter impedance.
(2) Current measurement
On the contrary, an ideal ampere meter should have resistance as less as possible. Thus, the Rshunt is used to connect with the galvanometer in parallel fashion to reduce the meter impedance and the jack “A” in the figure is used for this kind of measurement.
(3) Resistance measurement
As you may see in the figure, the resistance measurement use the same jack with the voltage measurement but the connection is totally different. In this case, the galvanometer is connected with a battery, a series resistor, and the measuring device in series fashion. Since the voltage is fixed by the battery, the galvanometer senses the resistance from the current through the meter and the measured value is calibrated to the resistance value.