Low melting point.
Explanation
An electric fuse is a safety device used in electrical circuits to protect appliances from damage caused by excess current. It operates on the heating effect of electric current (Joule’s Heating), where the heat produced is given by \( H = I^2Rt \). The fuse wire is designed to melt and break the circuit when the current exceeds a specific safety limit.
Analysis of Properties
- Low Melting Point: This is the most critical property of a fuse wire. When the current flowing through the circuit exceeds the rated value, the temperature of the fuse wire rises rapidly. A low melting point ensures that the wire melts immediately, breaking the circuit and stopping the flow of current before it can damage the appliance or cause a fire. Common materials used are alloys of tin and lead.
- Resistance: While not explicitly the sole focus of the correct option, a fuse wire generally has a higher resistance (or resistivity) compared to the connecting wires (like copper). This ensures that heat is generated primarily in the fuse wire rather than the rest of the circuit. However, among the given options, the low melting point is the defining characteristic for its operation as a safety break.
- Ferromagnetic Nature: The magnetic properties of the material are irrelevant to the thermal operation of a fuse.
Option Analysis
- High melting point and high resistance. is incorrect: A high melting point would prevent the fuse from melting during a current surge, rendering it useless as a safety device.
- High electrical conductivity and high melting point. is incorrect: Similar to High melting point and high resistance., a high melting point is undesirable.
- Low melting point. is correct: A low melting point is essential so that the wire melts at a specific temperature threshold corresponding to the current limit.
- Ferromagnetic nature. is incorrect: Ferromagnetism is not a required property for a thermal fuse.
Key Takeaway: The primary function of an electric fuse is to break the circuit during an overload; therefore, it must possess a low melting point to melt quickly when the temperature rises due to excess current.