How can the range of an ammeter be increased?

To measure larger currents than the basic meter movement can handle, you must provide an alternative path for the excess current. This is achieved by adding a low-value resistor, called a shunt, in parallel with the meter. Because the shunt is in parallel with the meter movement, it takes the bulk of the current, leaving only a small, safe, and proportional amount to flow through the actual meter coil. The meter's scale is then calibrated to reflect the total current flowing through the combination. Adding resistance in series with the meter would convert it into a voltmeter. Adding resistance to the circuit under test would reduce the actual current flowing in the circuit rather than expanding the meter's measurement capability.