Owen Llewellyn describes how the genius of James Watt and the skill of Brixton millers contributed to the production of high-quality flour

Image: Penny Steele

When they climb the stairs to the first floor of Brixton Windmill (traditionally known as the meal floor), visitors are instantly intrigued by the sight of long arcs of black-painted cast iron, suspended from the joists.

This “centrifugal governor” consists of two substantial spherical masses, called flyballs, on sturdy metal arms radiating from a central axle that disappears into the stone floor above.

As part of renovations from the 1960s onwards, it’s likely that the existing governor was salvaged from another mill rather than being the original. It has an elegant design, with ornamental touches that hint at an age when both form and function were considered important in mechanical engineering.

Watt's innovation

The centrifugal governor we see in the mill today was a relative newcomer in the early 19th century. Invented by Christian Huygens in the 17th century, the design was developed in the 1780s by renowned scientist and inventor James Watt and incorporated into many industrial applications.

Its purpose in a windmill was to regulate the separation of the millstones to avoid friction when a high wind on the sails was turning the runner stone too fast.

The adjustment of the millstones is called ‘tentering’, a process that was very important to a miller. Careful adjustment of the millstones was necessary for maintaining high-quality flour. In addition, one or both stones might be damaged and cause the suspension of milling until costly repairs could be made.

Most dramatically, the excessive friction could scorch the grain and flour, or create a spark which might lead to a devastating ignition of the very fine flour dust in the air, destroying the mill and the miller with it – not uncommon in the history of windmills.

A miller would perform this task by adjusting a screw on the gear attached to the governor, which would then automatically tenter the stones as the speed of the runner stone responded to the rise and fall of the wind.

A simplified design of the governor and tentering gear is shown below. A governor exploits the principle of the conservation of angular momentum, a fundamental concept in physics that can be found at play in everything from the movement of electrons to the orbits of stars around galaxies.

The classic example is that of an ice skater spinning slowly with arms stretched and then faster and faster as their arms are drawn closer to the body. The governor exploits this effect by having, instead of flexible arms, rigid ones that are allowed to rise and fall along the stone shaft as the rotational speed of the shaft varies.

The governor is attached to the tentering gear in such a way that, as the governor rises, the tentering gear forces the runner stone upwards, resulting in the increased separation of the stones, as illustrated by the animation below.

Tentering skills

Determining how much to adjust the tentering would be a key skill of the miller, and would greatly affect the quality of the flour produced.

To be commercially viable, the mill would have to process a variety of grains throughout the year, and each type of grain – even each new batch of a single grain – might well require a different setting. The skill of producing high-quality flour was not one acquired overnight.

Urbanisation in the environs of the mill during the 19th and 20th centuries has taken the wind out of Brixton Windmill’s sails and left the governor idle. Now the mill is a reminder of the time when Brixton was a rural village on the edge of a great city.

All milling at Brixton Windmill is now carried out using the electrically powered modular mill, which shares the meal floor with the centrifugal governor. Of course, the modular mill itself needs to be tentered by our dedicated band of millers to produce the high-quality flour we now offer – though the mechanism for this is hidden under the machinery.


At first glance the governor may appear to be an elegant but redundant symbol of a lost age. However, it could equally be seen as a humble example of the exploitation of a fundamental universal process, and one that was key to improving the quality of one of our staple foods.

An early example of automation in food manufacture, it still left room for skill and craftsmanship by its operators, the millers who worked over the years at Brixton Windmill.