Hollow wooden-sounding box 1290 x 130 x 110 mm, with mm scale, has two fixed bridges 72 cm apart over which wires are stretched. To one wire is fitted a spring balance reading to 100N x 5N and its tension can be adjusted by a winged nut and measured. Another wire passes over a pulley and can be loaded as desired. A third wire, for comparison, can have its tension adjusted by a wrest pin. With 3 movable bridges, wrest-pin key, spring balance, and set of four wires of different diameters, each 1.5 meter long, without masses.