Impulse: The moment of contact between the escapement and the balance which allows the power of the mainspring to be momentarily fed through to the balance, so that it keeps moving instead of drifting to a halt.

Impulse pin: In a lever escapement, a pin made of ruby, mounted on a steel disc called the roller which is fitted to and concentric with the balance-staff. This pin engages in the fork of a lever and receives impulses from it, thus keeping the balance moving. Also called roller jewel.

Incabloc: Incabloc is a trade name for a type of shock absorbing device/spring used to protect the delicate parts of the mechanical watch escapement. Mentioned here as it is probably the most widely used and some watch manufacturers used to draw attention to it by referring to it on the watch dial itself.

Index/Indices: Usually refers to the markings on the dial of a watch showing hours and minutes. Can however refer to the markings on the regulator of a watch movement to aid precision adjustment for accurate timekeeping.

Isochronous/isochronism: A oscillating object such as a balance-spring is said to be isochronous, or to show isochronism, when it takes exactly the same length of time to accomplish each swing or arc irrespective of the distance covered; thus it beats at the same pace whether it is being driven strongly or weakly. The ordinary flat balance-spring as introduced in the 1670s was only roughly isochronous (and has often grown even less so with time through metal-fatigue, especially in verge watches). Precision makers of the next century devised various special configurations to make this quality more complete; Breguet's overcoil was the most successful of these and the only one which survives today.

J

Jewels: In the mechanical watch and some quartz watches, jewels (usually made from synthetic ruby) are used as bearings for those parts of the movement subject to constant motion. They are not valuable at all in the monetary sense but they are valuable in aiding the precise running of a watch over a long period and reducing wear. It is a fallacy that the more jewels the better the watch. A basic handwound mechanical usually comprises 17 jewels which in the main is the optimum count. Automatics may require more for the winding mechanism itself. In the 1960s there appeared to be a competition to see who could fit the most jewels in a watch movement, manufacturers proudly referred to 100 Jewels on the dial; opening the watch usually revealed that up to 80% of these jewels had no purpose and were simply mounted here and there on the movement to up the jewel count.

Jumping Hour: System of timekeeping whereby the seconds and minutes are shown by traditional hands but the hour is shown in a dial cutout (often at 12), on the minutes hand reaching 59 minutes, the hour disc under the dial will jump to the next hour.

K

Karussel: A slow-turning variety of tourbillon devised by Bonniksen of Coventry (U.K.) in 1894.

Key-wound/key set: refers to a mechanical design requiring a watch to be both wound and set by the insertion of a key, usually fitting over a square shaped shaft. This concept was largely restricted to pocket watches, and usually prior to circa 1900.

Kif: A trade name for a shock absorbing system; in a similar vein to Incabloc.

L

Latitude: The angle from a point on the Earth's surface and the equatorial plane, measured from the center of the sphere. Lines joining points of the same latitude are called parallels, which trace concentric circles on the surface of the Earth, parallel to the equator. The north pole is 90° N; the south pole is 90° S. The 0° parallel of latitude is designated the equator. The equator is the fundamental plane of all geographic coordinate systems, dividing the globe into Northern and Southern Hemispheres.

LCD Display: Or Liquid Crystal Display; used for the display on most modern digital watches. Followed from the earlier LED or Light Emitting Diode display of the first quartz digital watches. The LCD was preferred as it used much less power than the LED thus the time could be shown constantly as opposed to having to press a button for time display.

Leaf: The word ‘teeth’, applied to the projections on a gear-wheel, is replaced by ‘leaves’ when speaking of a pinion.

Lever escapement: The standard escapement for domestic watches from about 1840 in Britain and the U.S.A. (1890 in Switzerland) until the rise of the quartz watch; it is still used in mechanical movements today. The ‘lever’ is a pivoted bar to which is attached an arm carrying two pallets, which alternately engage with the teeth of the escape-wheel; the other end of the lever is forked and engages with the impulse-pin which is mounted on a steel disc (the roller) centred on the balance-staff. The lever was invented by Thomas Mudge in 1757 and applied by a few 18th-century makers including Breguet; its use gradually spread after about 1815. It has two main forms: the side lever with the pallet-arm integral with and parallel to the lever (favoured by 19th-century British makers), and the straight-line lever in which the pallet-arm is at a right angle to the lever, preferred in America from about 1870 and afterwards universal.

Lever setting: A method of setting the hands by means of a lever (normally concealed under the bezel) which when pulled outward operates a clutch which releases the motion-work from the train, so that the hands can be pushed round manually without disturbing the running of the watch. This arrangement is found on many early stem-wound watches; pendant or stem setting was distrusted because it was felt that the crown might spring out or be pulled out accidentally, allowing the hands to get out of place.

Longitude: Position on the earth's surface eastward or westward of an agreed starting-point (meridian – usually the meridian-line at Greenwich Observatory, London), expressed in degrees, minutes and seconds. Identifying a ship's longitude when out of sight of known landmarks was a lasting problem, beset by so many variables that it could scarcely be done by observation alone. The simplest method was to carry on board an accurate timekeeper which showed the time at the chosen meridian, and compare it with the time at the ship's location as determined by observation of the sun; the difference between the two could be reliably translated into terms of distance, but the clock had to be of outstanding precision and reliability, since an error of only four seconds could represent as much as a mile if the ship were near the equator. John Harrison's fourth chronometer (1759) was the first practical solution.

Lugs: Protrusions on the case of a watch to which the bracelet or strap is fitted. Various types of lugs can be found such as rounded lugs, teardrop lugs and hidden lugs. The term is derived from an old English slang term for "ears".