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.

Since 1889, an international prototype metre (a bar made of a platinum-iridium alloy with a low coefficient of expansion) has been used to define the metre. Since 1983, the metre has been defined as the distance travelled by light in a vacuum during 1/299,792,458 of a second. Thus, length is defined by a temporal measurement.

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

In watchmaking, many tools are used to measure length: rulers, callipers, micrometres, comparators, gauges, or optical tools (laser) are the main ones.

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

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.

For instance:

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

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

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