Which of the following is a desirable property of a fuse Wire?

Analyze the question: A fuse is a safety device placed in a circuit. If the current becomes too large, the fuse wire must heat up and break the circuit, protecting appliances and wiring from damage or fire.

Concept: The fuse wire works on the heating effect of electric current (H = I²Rt). When the current exceeds the rated value, the wire heats up enough to melt and break the circuit. So the wire must melt easily, which means it needs a low melting point. Fuse wires are usually made of tin-lead alloy, which has a low melting point.

Checking each option:

• A. High expansivity: Not relevant. Expanding does not break the circuit; melting does.
• B. High thermal conductivity: Undesirable. The wire would lose heat quickly to its surroundings and might not reach its melting point when it should.
• C. Low density: Has no bearing on how the fuse operates.
• D. Low electrical resistivity: Not desirable. A fuse needs enough resistance to heat up. Very low resistivity would make it heat poorly, so it would be slow to blow. (Fuse wire ideally has relatively high resistance for its size, along with a low melting point.)
• E. Low melting point: Correct. The wire melts quickly when excess current flows, opening the circuit.

Answer: E, Low melting point.

Tip: Remember "a fuse must be the weakest link": it should be the first part of the circuit to melt. Whenever you see "fuse wire" in an exam, think of low melting point (and a suitable, relatively high resistance).