A measuring instrument consisting of an enclosure containing resistance coils with convenient plugs or switches for placing coils of various resistance in and out of a balancing circuit is called a?

• A. Voltmeter
• B. Ohmmeter
• C. Current transformer
• D. Wheatstone bridge

Answer: (D)

Balancing a bridge to compare resistances is what this instrument relies on. A Wheatstone bridge uses four resistive arms arranged in a diamond, with a galvanometer in the middle and a source across opposite corners. By plugging in different resistance coils, you create two voltage dividers. When the ratio of the resistances in one leg equals the ratio in the other, no current flows through the galvanometer—this is the balance point. At that moment you can solve for the unknown resistance using the known resistance values, because the two divider ratios are equal.

This description matches a Wheatstone bridge, where the coils and adjustable connections let you place precise known resistances into the circuit and adjust until balance is achieved. It isn’t a voltmeter, which directly measures voltage; it isn’t an ohmmeter, which determines resistance by applying a current and reading the response; and it isn’t a current transformer, which changes current levels in a circuit. The key idea is using a balanced bridge to deduce an unknown resistance from known ones.