LENGTHS

The metre is the official unit of length measurement in the International System of Units (SI). It was initially defined as representing one ten-millionth of a half meridian.

Definitions of the metre

From 1889, a standard metre (a bar made of a platinum-iridium alloy with a low coefficient of expansion) was used to define the metre internationally. Since 1983, however, the metre has been defined as the distance travelled by light in a vacuum over 1/299,792,458th of a second, which is equivalent to about 300,000 kilometers per hour. Length is thus defined by a temporal measurement.

Units in watchmaking

The small dimensions of watch mechanisms and the extreme precision required for their manufacture have established the millimetre (mm) as the basic unit of length throughout the watchmaking world. Depending on the activity and the components concerned, using the thousandth of a millimetre (micrometre or micron, μm) is now common when it comes to dimensions and tolerances.

The ligne (ln or ‘’’) is an ancient unit of length measurement that endures in the watchmaking world. Originating in the Middle Ages, this unit was equivalent to one-twelfth of an inch. Over time, it has seen slight variations in length. The one used in watchmaking represents 2.255 mm. Today, “ligne” is still used at times, but exclusively to designate the diameter of a movement, also known as its “calibre.” Thus, a calibre of 12 1/2’’’ has a diameter of 28.1875 mm.

Measuring tools for watchmaking

In watchmaking, many tools are used to measure length. The most important of these are: rulers, callipers, micrometres, dial gauges, gauges, and laser optical tools are the main ones.

ANGLES

The measurement of angles in degrees originates from the Babylonians and their sexagesimal calculation base. Thus, a circle is divided into 360 sections of one degree each. A degree is further divided into 60 minutes of arc, which are themselves divided into 60 seconds of arc. Nowadays, the decimal division of the degree is commonly used as follows:

36.7534° (decimal division) = 36°45’20”24”’ (sexagesimal division)

The unit of angle is the degree (°). Minutes of arc are symbolised by (’), seconds of arc by (’’), etc.

In watchmaking, angles can be measured using a protractor, compass, gauge, or optical instruments. Of course, they can also be calculated using trigonometry.