A sonometer is a device used for studying transverse vibrations of a string. It is made up of a hollow box with a string attached to it. It is also called a monochord because it has only one string.
The string of a sonometer is at one end of the box and runs over it to the other end. Weights are added on the loose end of the string to produce tension in the wire. The device demonstrates the relationship between the tension of a string, the density of that string and the frequency of the sound produced when it is plucked.