Desaguliers' tribometer is an instrument for the study of friction that takes its name from that of its inventor: John Theophilus Desaguliers (1683-1744), a professor of physics and renowned lecturer and demonstrator at Oxford and at the Royal Society in London.
It consists of a light brass wheel (diameter 14 cm) at the centre, the horizontal axis of which, instead of lying on fixed supports, lies freely on both sides on the crossed outside of two mobile brass wheels with horizontal, non-coaxial axes (diameter 7.5 cm).
Thanks to this arrangement, conceived by the genial watch-maker Henry Sully (1680-1788), in which the axis of the central wheel transmits its motion to the two pairs of lateral wheels; instead of producing sliding friction, as would occur if it lay on fixed supports, it produces only rolling friction and the motion takes place with no appreciable dissipation of energy.
In the rest position the central wheel is kept raised by putting the ends of its axis in empty cylindrical supports.
The instrument is made entirely of brass and its rectangular base, five millimetres thick (width 16 cm, depth 4 cm) is fixed on a square box (sides 24 cm, height 6 cm) of polished wood with a drawer for storing accessories and samples to be studied.
Two small rods lying on the axis of the central wheel can be weighted down with light weights at one end, while on the other they are fixed to a support held by a column 12.3 cm in height by means of a horizontal pin that leaves it free to rotate.
The instrument is put into movement by a spring that has one end attached to the axis of the central wheel and the other to the frame of the instrument.
It is possible to increase the wheel's moment of inertia by applying four small, identical brass balls on pins on the outside of the wheel by pressure.
In the same way, it is possible to study the inertia produced by the air by applying four identical brass vanes instead of the balls.
The instrument is protected by a glass case.
An identical apparatus appeared in the catalogue of the watchmaker Deleuil in Paris in 1863.

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