Tourbillon serves to neutralise the effects of gravity and was patented in 1801 by Abraham-Louis Breguet.

A tourbillon encloses within its cage all the components of the escapement, the balance wheel, the balance spring, the pallets and the escape-wheel, all placed on the axis of the seconds wheel. For a large part of the tourbillons, the pinion of the seconds wheel will be integral with the cage and its wheel integral with the plate. The aim is to make all this rotate on its axis, generally in 60 seconds so that a second hand can be placed on the cage.

Any mechanical watch that does not have a tourbillon and has a flat balance spring is affected by gravity. However, it is not the whole watch that is affected. It is not the whole movement either. It is not the entire escapement either. It is basically only the flat balance spring that is affected by gravity and it is mainly affected in the vertical positions and not in the horizontal positions. This is much more relevant to pocket watches that remained in a vertical position in the owner’s waistcoat than to wrist watches that are in different positions when worn. Some say, however, that the tourbillon also makes sense on wristwatches equipped with folding clasps, which also sometimes remain upright on a bedside table overnight.

The principle of the tourbillon can be simply demonstrated with a bicycle wheel with its valve serving as a bottom-heavy. When the wheel is in a horizontal position the weight will not affect the position of the wheel. When the wheel is in a vertical position the weight will affect the position of the wheel. It is this phenomenon that causes a watch with a flat balance spring to be affected by gravity in the vertical position but not in the horizontal position.

A flat balance spring is therefore affected by this imbalance. A balance spring with an ascending terminal curve is much less affected by the effects of gravity because its centre of gravity is much more centred in its beat than the flat balance spring. So integrating an ascending balance spring into a tourbillon is a bit like wearing trousers with a belt and braces, it makes little sense.

If we analyse this balance defect, therefore on a flat balance spring and in a vertical position and in the four positions 90 degrees apart, we will see a running defect. If we put a watch with a flat balance spring in a vertical position for a whole night without moving it, the watch will be affected by gravity for the whole time. If this flat balance-spring was inside a tourbillon and placed on the same piece of furniture for the same period of time, the watch would obviously not move but its escapement would constantly turn on the tourbillon axis. Thus the effects of gravity are neutralised.

Most tourbillons are equipped with a Swiss lever escapement, but there are also some rare models equipped with a detent escapement, which has a detent inside the cage instead of the lever. Also, one can sometimes find flying tourbillons that do not have a bridge above the tourbillon.