There are a number of ways of exciting standing waves in ropes
and springs using non-commercial vibrators such as
loudspeakers,1 jigsaws,2 motors,3 or a simple tuning fork,4
including the rhythmical shaking of a handheld Slinky. We have
come up with a very simple and cheap way of exciting stationary
waves in a string, which anyone, particularly children, can try
at home. It consists of using an electric toothbrush to produce
a regular sideways motion that can be easily transmitted to an
elastic cord. Most suitable for this experiment is the kind of
unit that has a metal rod protruding from the front (see Fig. 1,
which shows our Braun Oral-B®) to which a brush is normally
affixed. The ends of the cord are attached to stands. Elastic
cords that come with some school notebooks work well for this
experiment, but a longer cord allows the observation of a larger
number of modes. The toothbrush unit is placed near one end of
the cord, which is wrapped once around the rod. As the
toothbrush vibrates at a fixed freque