(2) The d'Arsonval galvanometer can be modified to measure either
amperage or voltage in a circuit. When a low-resistance wire (a shunt) is connected in
parallel with a galvanometer that has an appropriately calibrated scale, it serves as an
ammeter; when a high-resistance wire is connected in series with the galvanometer, it
serves as a voltmeter. To measure the voltage and current of AC, additional
modifications are required, but the same principle is used in AC meters.
b. The voltmeter is a high-resistance galvanometer used to measure voltage. It
measures the potential difference across any two points in an electric circuit and must
always be connected in parallel (V, figure 2-15); that is, connected by means of a wire
to either side of the device across which the voltage drop is to be measured.
Figure 2-15. Placement of voltmeter and ammeter in circuit. The
voltmeter (V) is connected in parallel with the device
across which the voltage is to be measured (in this case
the battery). The ammeter (A) is connected in series in
the circuit so that all current passing through the circuit
must pass through and be measured by the ammeter.