What is the common device used to increase the current measuring range of an ammeter?

The use of a shunt is pivotal in increasing the current measuring range of an ammeter. A shunt is a low-resistance component placed in parallel with the ammeter. By allowing most of the current to pass through the shunt, it reduces the amount of current that needs to be measured by the ammeter itself, which protects the ammeter from exceeding its maximum current rating. This enables the measurement of higher currents without damaging the meter.

In practice, when the current flows through both the shunt and the ammeter, the shunt offers a path of lower resistance, allowing the ammeter to measure a fraction of the current. The voltage drop across the shunt is proportional to the current flowing through it, and by using Ohm's law, it can be calculated to yield the actual current flowing through the circuit.

The other devices listed serve different functions. A transformer is typically used for voltage transformation rather than directly increasing current measuring capabilities. A resistor can limit current, but it doesn't assist in measuring higher currents effectively. A fuse is a safety device that interrupts the flow of current in case of overloads but does not play a role in extending the measurement range of an ammeter.