__________ is used as fuse wire in thermite welding.

Explanation

**Step-by-Step Solution:**

1. **Understanding Thermite Welding**: Thermite welding is a process used to join metals using a chemical reaction between metal powders. It is often used for welding railway tracks and heavy machinery.

2. **Identifying the Fuse Wire**: In thermite welding, a fuse wire is needed to initiate the reaction. The fuse wire must be a material that can burn brightly and produce enough heat to start the welding process.

3. **Choosing the Right Metal**: The metal that is commonly used as a fuse wire in thermite welding is magnesium.

4. **Reason for Using Magnesium**: Magnesium is chosen because it burns with a dazzling light and produces a significant amount of heat when ignited, which is essential for the thermite reaction to occur effectively.

5. **Final Answer**: Therefore, the answer to the question is: **Magnesium** is used as fuse wire in thermite welding.

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