For people who are not familiar with the concept, an ammeter is a device that measures electric current. The circuit diagram below will show how to convert galvanometer into an ammeter.

In the figure below, the galvanometer is connected to a battery and a resistor. The battery discharges through the galvanometer and charges up in the resistor.

The voltage across both of these components is proportional to the current flowing through them, but since there is only one resistor there will be no voltage across it at any point in time. This means that it will always be at zero volts and therefore we can say that its resistance is infinite which means it has no voltage drop across it at any point in time. We can use this property of infinite resistance of resistors to measure current by measuring their power or power factor.