What is the characteristic of ideal ammeters?

An ideal ammeter is defined by its key characteristic of having no resistance. This is essential because an ammeter is used to measure electric current in a circuit, and if it possessed any resistance, it would introduce a voltage drop in the circuit, thus affecting the current being measured. The goal of using an ideal ammeter is to ensure that it does not alter the circuit dynamics while providing an accurate current reading.

By having no resistance, an ideal ammeter allows current to flow freely through it, ensuring that the measurement reflects the actual current running through the circuit. In practical applications, real ammeters strive to have as low a resistance as possible to mimic the behavior of an ideal ammeter.

The other characteristic options, such as infinite resistance or variable resistance, would interfere with the current measurement. Despite ammeters being able to measure both alternating current (AC) and direct current (DC) in practical devices, the defining feature of an ideal ammeter remains having no resistance.