What instrument is used to measure very low currents, such as those in bridge circuits?

The instrument designed specifically for measuring very low currents, particularly in sensitive applications like bridge circuits, is indeed the galvanometer. Galvanometers are highly sensitive devices that can detect and quantify minute electrical currents. They work on the principle of galvanomagnetic effects, which means that they can provide an accurate reading even with the smallest fluctuations in current.

Bridge circuits often require precise measurements to ensure accurate balance and functionality, which is where a galvanometer excels. It can show very small deflections, indicating slight changes in current, allowing for high precision in measurement applications.

In contrast, a current clamp is typically used for measuring higher currents, employed in electricians' work to measure alternating and direct currents without needing to disconnect circuits. A millimeter (which likely refers to a multimeter) is a versatile testing tool that can measure voltage, current, and resistance, but might not have the sensitivity required for very low current measurements. A potentiometer, while useful for measuring voltage and calibrating circuits, is not designed for current measurements at all.

Thus, the galvanometer stands out as the correct choice for its specific function in measuring low currents, making it essential for precise applications in bridge circuits and other similar setups.