Conversion of Galvanometer into Ammeter and Voltmeter

A bare galvanometer coil, exactly as constructed in Section 4.13, is a delicate instrument: its

fine wire can safely carry only a very small current (typically in the milliampere range, called its

full-scale deflection current, $I_g$) before being damaged, and it has its own small internal

resistance $G$ (the coil's own resistance). To turn this delicate, small-range instrument into a

practically USEFUL ammeter (for measuring a much larger current) or voltmeter (for measuring a

potential difference), one additional, carefully chosen resistor must be added.

Conversion to an ammeter (a low-resistance shunt, in parallel). To measure a current up to some

larger value $I$ (with $I > I_g$), a low-resistance shunt $S$ is connected in PARALLEL with the

galvanometer coil. The idea is that the shunt diverts most of the current $I$ around the delicate

coil, letting only the safe current $I_g$ pass through the coil itself, while the shunt carries the

remaining $(I-I_g)$. Since the shunt and the coil are in parallel, they share the SAME potential

difference across them:

An ammeter must be connected in SERIES with the part of a circuit whose current is to be measured,

and, for it to disturb that current as little as possible, its OWN net resistance should ideally be

as close to ZERO as practically possible; the parallel combination of the low shunt $S$ with the coil

resistance $G$ indeed gives a net resistance LOWER than $G$ alone, exactly the direction needed for a

good ammeter.

Conversion to a voltmeter (a high series resistance, in series). To measure a potential

difference up to some larger value $V$, a high resistance $R$ is instead connected in SERIES with

the galvanometer coil. The same safe current $I_g$ now flows through BOTH the coil and the added

series resistor together, so the reading $V$ corresponds to the TOTAL potential drop across the

whole series combination:

A voltmeter must be connected in PARALLEL across the circuit element whose potential difference is

to be measured, and, for it to draw as little current away from that element as possible (so as to

disturb the very quantity it is trying to measure as little as possible), its OWN net resistance

should ideally be as HIGH as practically possible; adding the large series resistance $R$ achieves

exactly this. …