# How a Fuse Works

My physics classes are studying electricity.  One of the demos I do for electricity is to show how a fuse works by exploding a strand of wire.

For the demo, I made a “fuse” holder by taking the knife out of a knife switch and replacing it with a second jaw.For the “fuse,” I placed a single strand from 26-strand 16 AWG copper stranded wire into the above holder, in line with the light bulb and a fully-functional knife switch. According to the calculator on Cirris’s Model of a Wire Fusing in Free Air page, it would theoretically require a current of 10.2 A to melt this strand of wire.

For my circuit, I placed my “fuse” in line with a (normally functioning) knife switch and a 15 W incandescent light bulb.  When I plugged the circuit in and closed the knife switch, the bulb illuminated, as expected.  A 15 W bulb has a resistance of about 850 Ω, and draws a current of about 0.13 A when connected to 110 VAC, which is much less than the current needed to melt the wire.

Then, I opened the switch and clipped a wire across the light bulb socket, creating a short circuit.  When I closed the switch, the “fuse” blew.  (I.e., the strand of wire exploded spectacularly.)

One of my students took a phone video of the demo.  The pictures shown below are the two frames immediately after the bright flash, showing the trails of white-hot copper flying out from the “fuse”.

According to MS Movie Maker, the frames are 1/100 of a second apart, so these images would be 10 ms and 20 ms after the fuse blew.

Needless to say, now that I’ve seen these pictures, I will wear safety goggles when I perform this demo in the future!