The instrument used for detecting the presence of electric current in a circuit is

A galvanometer is a sensitive electromechanical instrument specifically designed for detecting and indicating the presence of an electric current in a circuit. It operates on the principle of electromagnetic induction, where current flowing through a coil in a fixed magnetic field produces a rotary deflection of a pointer. While an ammeter is used to quantify the magnitude of current, the galvanometer serves as its core component, capable of showing both the direction and presence of even small amounts of current. In contrast, other options serve different purposes: a refractometer measures the refractive index of substances, a viscometer measures the viscosity of fluids, and a diffractometer is used in X-ray crystallography to study the structure of materials. Therefore, the galvanometer is the standard tool for current detection in electrical circuits.