Every fact

146 facts, every one sourced

Every fact on this site in one list, each linked to its source. Our build stores the source text and checks that the key words and figures in each fact really appear in it; see the sources page.

1 A clock is a machine that links something moving in equal, regular steps to a counter that adds up the steps. Britannica
2 Every clock, 12-hour or 24-hour, works on that same principle. Britannica
3 NIST puts it the same way: every clock needs a regular, repeating action to mark off equal bits of time, and a way to count them and show the result. NIST
4 Modern clocks are regulated by a balance wheel, a pendulum, a vibrating crystal or the workings of atoms. NIST
5 The history of timekeeping is the search for ever steadier actions to set a clock’s rate. NIST
6 A gear is a toothed wheel on a turning shaft. Britannica
7 Gears work in pairs: the teeth of one mesh with the teeth of the other, passing on turning motion and turning force without slipping. Britannica
8 Most gears are round, and round gears keep the speed ratio between their shafts constant. Britannica
9 Gears with teeth on non-round shapes make the speed ratio change as they turn. Britannica
10 The smaller gear of a pair is called the pinion. Britannica
11 If the small pinion drives a bigger gear, the pair slows the motion down and boosts the turning force; drive it the other way and it speeds up but turns more weakly. Britannica
12 A driven gear with twice the pinion’s teeth turns with twice the force at half the speed. Britannica
13 To run smoothly, gear teeth must be cut to a carefully chosen shape. Britannica
14 Gears on parallel shafts can have straight teeth (spur gears) or twisted, screw-like teeth (helical gears). Britannica
15 Shafts that meet at an angle use bevel gears, with teeth on cut-off cones. Britannica
16 A worm looks like a screw, and its gear like part of a long nut bent round a cylinder. Britannica
17 Because a worm is basically a screw it can have just one tooth, which makes a worm and gear good for a big speed change in one step. Britannica
18 A spur or helical pinion needs at least five teeth to stay in mesh all the time. Britannica
19 For a big ratio between parallel shafts you may need several gear pairs in a row, called a train. Britannica
20 The most common angle between shafts that are not parallel is a right angle. Britannica
21 In a medieval-style watermill, a series of gears carries the stream’s power from the waterwheel to the millstone. Britannica
22 A clock’s wheelwork, or train, is the row of gears that carries motion from the weight or spring, by way of the escapement, to the hands. Britannica
23 The wheels and pinions must be made accurately so the power flows as steadily as possible. Britannica
24 Power enters through the main, or great, wheel, which turns a pinion, whose rod carries the next wheel, and so on down to the escapement. Britannica
25 In clockmakers’ words, a pinion is a gear with few teeth, and an arbor is the turning rod the gears are fixed to. Britannica
26 The gear ratios are chosen so that one arbor turns once an hour and can carry the minute hand. Britannica
27 A simple 12-to-1 gearing, the motion work, drives the hour hand from the minute hand. Britannica
28 A slipping clutch on the minute-hand arbor lets you set the hands. Britannica
29 The frame of a simple weight-driven clock is two plates held apart by four pillars. Britannica
30 The driving weight hangs from a line wound round a barrel, raised by turning the winding square or by pulling the line. Britannica
31 A crutch fixed to the pallet arbor ends in a fork that holds the pendulum rod. Britannica
32 The cannon pinion, which drives the hands, turns once an hour. Britannica
33 In a modern watch, the gears between barrel and escape wheel step the speed up about 4,000 times, using four pairs of gears. Britannica
34 Each pair in a watch usually steps up between 6 and 10 times, with pinions of only 6 to 12 leaves (teeth). Britannica
35 An escapement lets motion happen in controlled steps. Britannica
36 In a clock or watch it controls how energy passes from the power source to the counting mechanism. Britannica
37 The real innovation of the weight-driven clock was the escapement, which handed the pull of the falling weight on to the counting gears. Britannica
38 The escapement is what makes the tick-tock of a weight-driven clock. Britannica
39 The verge escapement, the classic early form that made the mechanical clock possible, was probably invented in 13th-century Europe. Britannica
40 It has a crown-shaped wheel, driven by a weight, whose teeth are stopped in turn by a pair of metal pallets. Britannica
41 The pallets sit on an upright rod, the verge, topped by a crossbar called the foliot with two small weights; moving the weights outwards slows it down. Britannica
42 The verge-and-foliot was the most common escapement. Britannica
43 Versions of the verge and foliot ruled for more than 300 years. NIST
44 Their weak point: how fast they swung depended heavily on the driving force and on friction. NIST
45 The anchor escapement, invented in England in the 17th century, works with a pendulum and needs much smaller swings than a verge with a pendulum. Britannica
46 Its pallets are shaped like an upside-down anchor, in the same plane as the wheel. Britannica
47 NIST says William Clement began building clocks with the anchor, or recoil, escapement in London in 1671. NIST
48 Britannica says English clockmakers brought in the long, seconds pendulum with the anchor escapement in 1670. Britannica
49 The anchor was a big step up because it disturbs the pendulum less than the verge does. NIST
50 Later escapements were made as detached as possible, giving the swinging part energy while leaving it free to swing. Britannica
51 In a pendulum clock the escape wheel moves on one tooth for each double swing and gives the pendulum a push to keep it going. Britannica
52 An ideal escapement would push without getting in the way of the free swing, and push the same every time. Britannica
53 Edmund Beckett, later Lord Grimthorpe, invented the double three-legged gravity escapement for the great clock at Westminster, now known as Big Ben, installed in 1859. Britannica
54 It became the standard for really accurate tower clocks. Britannica
55 Most mechanical watches use the lever escapement, invented in England about 1755 by Thomas Mudge. Britannica
56 It leaves the balance free to swing, touching it only to give its push and to be unlocked. Britannica
57 The lever is by far the most important watch escapement today. Britannica
58 A pendulum is a weight hung from a fixed point so it can swing back and forth under gravity. Britannica
59 Pendulums regulate clocks because each complete swing, the period, takes the same time. Britannica
60 For small swings, the period depends only on the pendulum’s length, and hardly at all on how wide it swings. Britannica
61 A heavier bob does not change the period, as long as the length stays the same. Britannica
62 Make a pendulum longer and it swings more slowly. Britannica
63 A pendulum that takes one second per swing is about 39 inches (990 mm) long. Britannica
64 Make it just 0.001 inch (0.025 mm) longer and the clock loses about a second a day. Britannica
65 Clocks are regulated by raising or lowering the bob on a nut screwed along the rod. Britannica
66 A steel-rod pendulum clock loses a second a day for a temperature rise of about 4 °F (2.2 °C). Britannica
67 Clockmakers fought this with mercury-jar bobs, gridirons of brass and steel rods, and rods of the special alloy Invar, which barely expands. Britannica
68 Gravity is not equally strong everywhere, so the same pendulum swings faster at the poles and at low altitude than at the Equator or up high. Britannica
69 Britannica’s pendulum article says Galileo first noticed the steady period around 1583, comparing a swinging lamp in Pisa cathedral with his pulse. Britannica
70 Britannica’s clock article says about 1581. Britannica
71 NIST says Galileo was studying the pendulum as early as 1582, and sketched a pendulum clock but never built one before he died in 1642. NIST
72 Christiaan Huygens made the first pendulum clock in 1656. NIST
73 Some authorities credit Galileo with the pendulum clock and others Huygens. Britannica
74 Huygens made the period truly constant with a pivot that made the bob swing along a curve called a cycloid instead of a circle. Britannica
75 His early pendulum clock was out by less than a minute a day, a first; later versions by less than 10 seconds. NIST
76 Huygens described the pendulum as a clock regulator in his book Horologium (1658), and published Horologium Oscillatorium in 1673. Britannica
77 The same Huygens found the true shape of Saturn’s rings and one of its moons. Britannica
78 The new eight-day wall clocks had very heavy weights, and many fell off weak plaster walls. Britannica
79 So the case was stretched down to the floor, and the grandfather clock was born. Britannica
80 In 1721 George Graham got a pendulum clock to within a second a day by correcting for the rod growing and shrinking with temperature. NIST
81 The 1921 Shortt clock had two pendulums: a slave that did the work and gave the free master pendulum its gentle pushes. NIST
82 Siegmund Riefler’s clock of 1889 kept time to a hundredth of a second a day. NIST
83 The mainspring, which drives a watch, is a flat band of spring steel; winding it coils it tighter and stores energy. Britannica
84 Britannica says spring-driven clocks first appeared about 1450, probably in southern Germany or northern Italy, the first portable timepieces. Britannica
85 NIST credits spring-powered clocks to Peter Henlein of Nuremberg, between 1500 and 1510. NIST
86 Britannica says the first watches appeared shortly after 1500, with early ones made by Henlein, a locksmith in Nürnberg. Britannica
87 Early spring clocks ran slower as the mainspring unwound. NIST
88 The dials of the first spring clocks usually had only an hour hand; minute hands were rare until the 1650s. Britannica
89 A mainspring pulls harder when fully wound than when nearly run down, a big problem for early watches. Britannica
90 The fix was the fusee: a cone-shaped, grooved pulley linked to the spring barrel by gut, later a chain. Britannica
91 Fully wound, the chain pulls on the narrow end of the cone; as the spring runs down it pulls on a wider part, so the leverage grows and the force stays almost even. Britannica
92 Modern watches use a going barrel instead, which drives the gears directly. Britannica
93 Until about 1580 German watch movements were almost all iron; then brass came in. Britannica
94 Robert Hooke designed a watch with a balance spring in the late 1650s, but there is no evidence it was a spiral. Britannica
95 Britannica says Huygens was probably first to design a watch with a spiral balance spring, in 1674–75. Britannica
96 NIST says Huygens developed the balance wheel and spring around 1675, letting watches keep time to 10 minutes a day. NIST
97 The balance spring does for the balance wheel what gravity does for a pendulum: it pulls it back so it swings to and fro. Britannica
98 Good watches run their balance pivots in jewels; the first patent for watch jewels was taken out in London in 1704. Britannica
99 The first self-winding pocket watch patent was taken out in London in 1780; Louis Recordon’s self-winding wristwatch followed in 1924. Britannica
100 Quartz clocks beat pendulums because they have no gears or escapements to upset their steady vibration. NIST
101 One of the oldest water clocks was found in the tomb of the pharaoh Amenhotep I, buried around 1500 BCE. NIST
102 The Greeks called water clocks clepsydras, “water thieves”. NIST
103 The Tower of the Winds in Athens, built in the first century BCE, had a 24-hour water clock. NIST
104 Water flow is very hard to control exactly, so water clocks could never be really accurate. NIST
105 The Antikythera mechanism, an ancient Greek bronze device for showing the movements of the sky, was recovered in 1901 from a shipwreck near the island of Antikythera. Britannica
106 It is dated to about 100 BCE, give or take 30 years. Britannica
107 X-ray images showed 30 gear wheels inside the fragments. Britannica
108 Nothing geared that complicated is known again until medieval cathedral clocks, a thousand years later. Britannica
109 It was made of bronze sheet and was probably in a case about the size of a shoebox. Britannica
110 A hand-turned shaft is thought to have driven the main gear, one turn for each solar year. Britannica
111 Its Moon gearing used epicyclic gears and a pin-in-a-slot to copy the Moon’s uneven speed across the sky. Britannica
112 A small dial showed when the Panhellenic games, including the ancient Olympics, were due. Britannica
113 Its exact purpose is still unknown. Britannica
114 Su Song (1020–1101) built an armillary clock in a tower, powered by a waterwheel and a chain drive. Britannica
115 Britannica gives the tower as 35 feet (11 m) tall. Britannica
116 NIST says it was over 30 feet tall and built in 1088 CE. NIST
117 It used a water-driven escapement invented around 725 CE. NIST
118 Its mechanism used ideas not seen in Europe for hundreds of years. Britannica
119 No one knows where the all-mechanical escapement clock came from; the first may have rung bells calling monks to prayer. Britannica
120 The first mechanical clocks clearly recorded were big weight-driven tower (turret) clocks that struck the hours, with no hands or dial. Britannica
121 NIST says big mechanical clocks appeared in the towers of Italian cities in the first half of the 14th century. NIST
122 Britannica names the oldest known mechanical clock as one added to Dunstable Priory in Bedfordshire in 1283, which had no face. Britannica
123 The oldest surviving clock in England is at Salisbury Cathedral, from 1386. Britannica
124 A clock put up at Rouen in France in 1389 still survives. Britannica
125 These early clocks may have been out by as much as half an hour a day. Britannica
126 On 22 October 1707 five of Admiral Shovell’s ships struck rocks off the Isles of Scilly, and more than 1400 men died. RMG
127 Spain set up the first longitude reward scheme, in 1567. RMG
128 Britain’s Longitude Act became law on 9 July 1714, offering a public reward for finding longitude at sea. RMG
129 John Harrison was a carpenter who taught himself clockmaking. RMG
130 His precision longcase clocks of the mid-1720s kept time to a second a month. RMG
131 He aimed for a portable clock that kept time to within three seconds a day. RMG
132 He brought his first sea clock, H1, to London in 1735; its two linked swinging balances made it ignore a ship’s motion. RMG
133 Returning from Lisbon in 1736, Harrison correctly said a headland was the Lizard, not the Start: the ship was 60 miles off course. RMG
134 He worked on H3 for 19 years. RMG
135 His clocks correct for temperature and, with clever anti-friction parts, run without any oil. RMG
136 H4, finished in 1759, is Harrison’s prize-winning longitude watch. RMG
137 H4 has a 102 mm dial and weighs 1.45 kg. RMG
138 H4 ticks five times a second: its big balance beats faster and swings wider than a normal watch’s. RMG
139 On the 1761 trial to Jamaica, William Harrison used H4 to predict landfall at Madeira earlier than the crew expected. RMG
140 NIST says the chronometer kept time on a rolling ship to about a fifth of a second a day, 10 times better than the prize needed. NIST
141 NIST says that by 1761 it had won the British government’s 1714 prize. NIST
142 Royal Museums Greenwich says the Board recommended only half the reward in 1765, the rest to wait until other makers could build similar timekeepers. RMG
143 When the Royal Observatory tested H4 for 10 months from 1766, it did not perform well, and the dispute flared again. RMG
144 Harrison was finally rewarded by Parliament, not by the Board of Longitude, and never got all he felt he was owed. RMG
145 Marine chronometers were expensive at first, so they spread slowly until simpler, cheaper designs arrived in the early 1800s. RMG
146 Royal Museums Greenwich displays the original H2, H3 and H4 and a replica of H1 at the Royal Observatory. RMG