Wave Machine

What you need

  • A roll of gaffer or duct tape. Duck or Elephant brands are the best.
  • Lots of wooden kebab skewers.
  • More jelly babies than seems reasonable.
  • Two tables or chairs of the same height. These will have tape stuck to them, so don't use good ones.
  • A ruler, or a number of fingers that are approximately the same width as a ruler.

For every metre of wave machine you will need about 20 skewers and at least 40 jelly babies.

What you do

  1. Stretch the gaffer tape between the furniture. It's probably best to try this with a 2 metre span the first time. You need to stretch the tape taut, which means sticking it down firmly - go right round the table or chair if you can but be careful of the varnish!
  2. Starting from one end, stick kebab skewers across the tape so they stick out evenly on either side. This is why you need good-quality tape: you'll need strong adhesive to hold the skewer. A damp day will ruin the adhesive, too. If your skewers fall off, this could be why.
  3. Space the skewers about a ruler's-width apart, about 5cm. The exact spacing isn't critical, so long as they're fairly evenly spread. Keep adding skewers until you've reached the end of the tape.
  4. Now put a jelly baby on each end of each skewer. You might think you'd only need twice as many sweets as skewers, but mysteriously they tend to disappear.
  5. Be careful when pushing the sweets onto the skewers. Be careful in case you accidentally poke your finger, but also be careful because you want the skewer to stay balanced - it should rest horizontally. You may need to push things around a bit to make sure your wave machine stays balanced all along its length.
  6. Once that's done, your wave machine is ready. Hold one of the skewers at one end, swiftly pulse it up and down, then let go. You should see the pulse travel the length of your tape, reflect off the far end, and return to the start. If you drive the end continuously, you can set up a standing transverse wave, too.

What's going on?

The tape acts as a torsion spring. A torsion spring is one that works by twisting: the more it is twisted the greater the force it exerts to try and return to its original shape. The tape in this experiment has a very low spring constant - you actually don't need very much force at all to twist it - and because of this a disturbance in one skewer will easily create a disturbance in the next. This is how the wave moves - or propagates - down the line. There is also very low damping: instead of the spring returning back to its original position immediately it will continue to twist back and forth for some time after the original disturbance has passed.

The jelly babies and kebab skewers provide a high angular momentum. They are reasonably heavy and so will not move particularly quickly up and down under the wave disturbance. The result is a wave that travels down the tape at a surprisingly slow speed, as the tape struggles to return to its central position.

With a little practice, you can demonstrate not just standing waves, but things like wavepacket dispersion too.

Special Safety advice

Be careful of fingers when stabbing the jelly babies onto the skewers.