Final answer:
An ammeter uses a low-resistance shunt in parallel with a galvanometer to maintain low voltage across the measurement device and protect it from high currents. A smaller shunt resistance is used for larger currents. Voltmeters are connected in parallel to measure voltage accurately.
Step-by-step explanation:
By placing a low-resistance shunt in parallel with a galvanometer, an ammeter maintains voltage across the measurement device to an acceptably low level. A small shunt resistance, typically a fraction of a milliohm, ensures that the ammeter has a very low overall resistance. When a high current needs to be measured, the ammeter uses a correspondingly smaller shunt resistance to prevent damaging the galvanometer by shunting most of the current through the shunt. In contrast, a voltmeter is connected in parallel with the device being measured, as objects in parallel experience the same potential difference, to accurately measure voltage without significant alteration to the circuit.