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From the 50th edition of the PH catalogue (1950):
The apparatus demonstrates Melde's experiment in a most effective manner and allows waves to be set up transversely in the string having the frequency of the vibrating rod. i.e: twice the frequency of the A.C. Supply.Turning the Apparatus through 90 degrees the experiment maybe repeated, the vibration being imparted along the string and having half the frequency of the A.C. Supply.The vibrating rod is actuated by a soft iron core in a coil carrying A.C. taken off a 12 volt transformer or from the mains through a lamp resistance.
This museum quality apparatus now some 60 Years Old is in fine working order and in its original box. This is a classic demonstration of experimental Physics up to the 1960's.
Wikipedia:
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 possition 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.
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