The Atwood machine, named after it inventor, George Atwood (1746-1807), a professor at Trinity College at Cambridge, who reported on it in 1784.
Rotation takes place with no appreciable friction since both ends of the axis of the pulley lie on the outside of two overlapping wheels having non-coaxial horizontal axes following the arrangement devised by the watchmaker Henry Sully and also used in the Desaguliers tribometer.
In the machine described here, which has an overall height of 2.85 m, the pulley (diameter 15.5 cm) and the two pairs of wheels (diameter 10 cm) on which its axis lies are assembled on a brass frame attached to a square, wooden platform (47.5 x 47.5 cm) veneered with walnut root placed at the top of two columns (height 2 m) of solid walnut. These columns are in turn attached to a sturdy square basement (61.5 x 61.5 cm) this too veneered with walnut root, with four levelling screws.
A brass plate attached to one of the two columns bears the words: Luigi Lusvberg Macchinista in Roma 1828.
In the groove of the pulley there is a silk string passing through two holes in the platform, at the ends of which there are two identical, perfectly balanced objects.
One of the two objects is placed on a metal disk (diameter 5.8 cm) that can be tilted and which is attached to the zero of a vertical graduated rule placed between the two columns.
Behind the two columns and the graduated rule there is a pendulum free to swing composed of a thick, brass-plated lens having a diameter of 16.3 cm and a suspending rod 106 cm long.
A lever attached to one of the columns holds the pendulum at rest in the position of the maximum range of oscillation.
If after placing another object on the one lying on the metal disk the pendulum lever is lowered, the pendulum begins swinging and, simultaneously, the disk overturns, freeing the two objects which fall vertically under the weight of the added object, with an acceleration given by the ratio between that weight and the overall mass of the three bodies in movement.
Falling times are measured by the pendulum which rings a bell when it begins oscillating; the distances travelled are measured on the graduated rule which is 1.895 metres long and has seventy divisions, each of which subdivided into twelve parts.
The acceleration of the fall can be varied by varying the mass of the overload. If this is small compared to the mass of the other two bodies, the acceleration will also be small.
Boutan - D'Almeida (1867), T. I, p. 17
Daguin (1878), T. I, p. 101
Drion - Fernet (1883), p. 29
Ganot (1864), p. 34
Segnini - Vergara Caffarelli (1990), p. 123