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# Protectors: How do Fuses Protect Wires From Overheating?

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Source:
Author: Janice VanCleave

### Problem

How do fuses protect wires from overheating?

### Materials

• 2 16-inch (40 cm) foil strips
• 2 clothespins
• duct tape
• large dinner plate
• 2 size D batteries (works best if new)

### Procedure

1. Insert the end of one foil strip into the jaws of one clothespin. Lay the strip across the top of the pin and secure with tape, as in the diagram. Repeat this procedure using the second foil strip and clothespin.
2. Pull a single strand of wire from the steel wool pad.
3. Stretch the strand of wire between the jaws of the clothespins. They should be as close as possible without touching.
4. Place the clothespins on the plate, turning them so that they are on their sides, and the foil strip on one pin is at the top and the foil strip on the second pin is at the bottom, as in the diagram.
5. Tape the two batteries together so that the positive terminal of one battery touches the negative terminal of the other.
6. Dim the lights in the room. While looking down at the strand of wire, touch the ends of the foil strips to the terminals of the battery column.
7. The results will be observed in less than 5 seconds. Immediately remove the strips from the battery terminal. WARNING: Holding the foil against the battery terminals for longer than 10 seconds can cause them to get hot and burn your fingers.

### Results

The strand of wire glows brightly, then breaks.

### Why?

The strand of steel wool acts like a bridge between the two strips of aluminum foil. With the bridge in place, electricity current can flow through the closed circuit. This flow of electric charges causes the foil and the steel wool strand to heat up. The temperature increase does not appear to affect the larger strips of aluminum foil. However, when a large enough current flows through the circuit, the strand of wire becomes hot enough to melt. In this way, the strand behaves like an electrical fuse. A fuse is a safety device that allows an electric current to flow through it, but if the current is too large, a wire in the fuse melts and opens the circuit. This break in the circuit stops the current. If too much current were to pass through a wire, it could get hot enough to cause a fire.

### Let's Explore

1. How would the amount of current flowing through the strand affect the results? Repeat the experiment twice. First use one battery, and then repeat again using three batteries. As the number of batteries increases, the amount of current pushed through the wire also increases.
2. Would the size of the steel wool strand affect the results? Repeat the original experiment twice. First use fine-grade steel wool, and then use coarse-grade steel wool. Use information from Experiment 15 to relate strand size to resistance of current flow.

### Show Time!

Circuit breakers (switches that automatically open an electric circuit when too much current is flowing) have replaced fuses in most modern homes. One example of this electrical protector has a switch made of two strips of dissimilar metal bonded together. When the bimetal strip is cool, it lays flat and bridges the gap between two points. If the strip heats up from an excessive flow of electricity, the metal on one side expands more than the metal on the other side. This causes the bimetal strip to curve, with the more-expanded metal forming the outside of the curve. When the strip curves, so that it no longer lays flat enough to form a bridge, electricity cannot flow through it. Simulate the curving of the bimetal strip by cutting one 2-inch × 8-inch (5-cm × 20-cm) strip and one 2-inch × 81/4-inch (5-cm × 2-cm) strip from poster board. Attach them at both ends with tape. Then, while holding both ends, push one side gently. The strip curves toward the side with the shorter piece.

### Check it Out!

1. Grounding is another way of preventing shocks and protecting appliances. Use an encyclopedia or ask an electrician to find out more about this practice. Explain why water pipe systems or metal rods driven into the earth are excellent grounding devices.
2. In 1746, Benjamin Franklin invented the lightning rod. Find out more about how this device protects buildings from lightning, which can produce a giant electric spark carrying up to several million volts. The electricity in your home ranges from 120 to 220 volts. Why is a lightning rod pointed on its top? What happens when lightning strikes the rod?