Understanding Instruments for Detecting Electric Current

The question asks about the instrument specifically designed to detect the presence of electric current within a circuit. Let's examine the options provided and their functions.

• Odometer: An odometer is an instrument used to measure the distance traveled by a vehicle. It is not related to detecting electric current.
• Galvanometer: A galvanometer is an electromechanical instrument that detects and indicates the presence of electric current. It is highly sensitive and can detect very small currents. When current passes through its coil, it creates a magnetic field that interacts with a permanent magnet, causing a needle or pointer to deflect. The direction of deflection indicates the direction of the current, and the magnitude of deflection can indicate the magnitude of the current (though ammeters are typically used for precise measurement of larger currents). Thus, it is primarily used for detecting the presence of current and also measuring small currents.
• Mercury meter: This term is not a standard name for an instrument used to detect or measure electric current. Mercury is sometimes used in thermometers or other types of meters, but not typically for direct current detection in this context.
• Cumeter: This term is not a recognized standard instrument for detecting or measuring electric current in a circuit.

Based on the functions of these instruments, the galvanometer is the instrument specifically designed to detect the presence of current in a circuit.

In summary:

Therefore, the instrument used to detect the presence of current in the circuit is a galvanometer.