Griffin Melde's Apparatus [vintage]
When an alternating electrical current is passed through a Melde's apparatus it causes an attached string to vibrate. The other end should be strung over a pulley with a weight tied to the end. If the pulley is placed at the correct distance from the Melde's apparatus for the frequency of the alternating current, then you will get a standing wave. It is called a standing wave because the two ends of the string are fixed in position and the points of the string which remain stationary also don't change position. This was done by Franz Melde to demonstrate that mechanical waves can experience interference patterns.