Old Passion's Clock Dictionary
Having a black finish that looks like ebony wood.
The plane of the Earth's orbit around the sun, passing through the center of the sun. Viewed from the Earth it is the apparent track of the sun throughout the year between the fixed stars. In a celestial or armillary sphere it is represented by a great circle passing through the equinoxes, and its plane is inclined at 23º 27' to that of the equator or 'equinoctial', the Earth's rotational axis being inclined at the same angle to the perpendiculr to the plane of its orbit.
A clock that runs for eight days on one winding.
A disc or wheel of artificial stone for grinding by machine. It is composed of emery grit bonded together, of even texture in various grades of fineness, and in various shapes and sizes. Rapidly rotated to grind many different metals the wheel can be run in water to keep it cool. A worn wheel may be cleaned or trued with a diamond truing tool.
The term 'American empire' applied to furnishings describes the style of c. 1825-40, which followed the Federal and preceded the Victorian. In clocks, this includes such types as bronze looking-glass, triple-decker carved, gilded and veneered, and some plain rectilinear cases. Among collectors the term 'Empire' case usually refers specifically to the two-door type of Connecticut shelf clock, having full rounded pillars flanking the larger two-section top door. The top section of this door enclosed the dial, and the lower section generally had a mirror and was flanked by mahogany-veneered corbel sections. The base of the clock was supported by turned feet dowelled into the bottom and the top splat was generally carved. Such cases were most frequently used with eight-day weight-driven brass movements, though a few are known containing wooden movements.
The process used to give a hard vitreous finish to clock dials and sometimes ornamental parts of clock cases. It should not be confused with painting. Enamel is a fusible vitreous silica-based compound which is mixed with water and applied to a sheet of copper or other metal and fused in an enamelling kiln, in which it melts to form a hard glossy surface. Obtaining the right thickness of enamel often calls for several coats, each applied and fired in turn after the previous coating has been rubbed down. Although normally opaque white, enamel can be colored with different metallic oxides; decoration such as numerals, signatures and other decoration on the dials, can be added in other colors and fused. The technique was extensively used for French clock dials, earlier ones being built up from individual enamelled plaques for each hour numeral.
In cloisonne enamelling, the area to be decorated is divided up by a design made in fine wire soldered to the ground metal and forming a series of compartments or cloisons, which are then filled with different colors of enamel and fired.
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In the past complex tools were often known as engines. Clock-barrel engines were used for cutting the spiral groove to take the gut lines of an eight-day longcase clock barrel. The tool was made rather like a pair of turns in which the barrel to be cut was centered, on its winding arbor, between a runner and a rotating or driving arbor. This arbor, which held the barrel winding square, was threaded along its length on which a block or carrier would travel axially as the work was turned. An arm extending from this block carried the hand cutter, and as the barrel was turned towards the operator pressure was applied to the cutter to form the spiral groove in the barrel.
The clock-barrel engine could also be used for cutting the groove in fusees for bracket clocks, except that the cutter would follow the conical profile of a fusee instead of the straight cylinder of a barrel. Because variation was often needed in the pitch of fusee or barrel grooves, a more elaborate form of engine was devised in which the pitch of the groove could be altered. In general terms it follows the same principle as the earlier engine, but in this case the cutter carrier, which is a flat rectangular bed free to travel axially, is moved by a lever from the guiding block on the driving arbor thread, this lever having a pivot which engages a slot in the carrier. By adjusting this pivot the amount of travel of the carrier can be varied; when the piot is nearer the front of the machine the tool carrier will travel less for a given number of turns of the work than when it is at the rear. By this means, and by having differently threaded driving arbors and blocks, considerable variety of pitch of fusee or barrel groove could be cut. Many modern Swiss fusee cutting tools use the same principle.
A wide variety of wheel-cutting engines have been made over the past three centuries, but all follow more or less the same general principle. It is generally agreed that the wheel-cutting engine evolved from manual methods of dividing a wheel in the 17th century, when revolving cutters to cut the separate wheel teeth spaces replaced hand filing, and when the blank to be cut was divided mechanically into the required number of teeth. For this purpose a dividing plate was used, on the arbor of which the blank wheel to be cut was mounted. The dividing plate was supported in a frame of various designs according to country and date, and a detent on the frame allowed the dividing plate, and thus the work, to be rotated at the chosen number of intervals required for cutting the chosen number of wheel teeth, according to the divisions of the circles on the dividing plate. As the blank was indexed round, therefore, a tooth space was cut by the rotating cutter, which was driven by an endless cord from a foot wheel or other power source. In some cases the cutter was advanced into and retracted from the edge of the blank, while in others the blank was advanced to the cutter. Early engines produced work which still required the teeth to be formed to their correct shape later with a rounding-up tool, but later machines were more advanced and had cutters which were shaped to produce the final tooth form, sometimes using three or more cutters mounted in a multiple cutting head for progressive work. Modern wheel cutting is normally done on a lathe with specially designed dividing heads, while in clock factories a cutter traverses a large number of wheel blanks mounted on a common arbor, cutting wheels in stacks.
Engraving produced by a machine. Beautiful decorative engraving in many designs can be done with the work in a specialized lathe or rose engine, in which the eccentric movement of the work, or cutting tool, or both, results in a repetitive pattern or moulding on the surface of the work. Such engraving is used to decorate clock dial centers and clock cases.
Engraving was used extensively in the decoration of clocks, for dial centers, backplates, cases, hour, minute and second numerals, and the maker's name. Numerals when engraved were usually filled with black wax to give them visual clarity. The usual method of engraving is to gouge or chisel out the surface design by hand using a sharpened tool called a 'graver' or 'burin', or a chisel and hammer. Engraving can also be performed by machine using a form of milling cutter and an engraving machine fitted with a pantograph, which can accurately copy a design.
The age of the moon on 1st January in any year; when this is known its age at any subsequent date in the year can be deduced. Since twelve lunations equal 354-36 days and a year equals 365.24 days, the epact increases by a little under eleven days from year to year, with a backward step of 29 1/2 days each time it passes this figure.
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The challenge of devising a clock which recorded solar as well as Greenwich mean time occupied the great makers of the golden age of English clockmaking, men like Thomas Tompion and Daniel Quare. Their less famous contemporary, Joseph Williamson, designed a number of equation clocks and also constructed several movements which Quare used. The probem was to compare mechanically solar time derived from a sundial, with mean time. Tompion worked out an equation table, which he had printed, to show the number of minutes and seconds solar time was fast or slow of mean time on any day of the year. An equation clock constructed by Daniel Quare had an auxiliary dial in the front of the case, below and separate from the main dial. It had two hands, a long one revolving once in 365 days and a shorter one with a sun to show the difference in minutes, fast or slow, between solar and mean time. Another of Quare's equation clocks had a double movement with two pendulums and, although it was not capable of recording accurately the equation of time, it was a reasonably acceptable solution.
A table giving the values of the equation of time throughout the year. Equation tables were often pasted inside the doors of longcase clocks during the late 17th and early 18th centuries.
Equation of Time
The difference between apparent solar time, i.e. the time shown by a clock running uniformly throughout the year. The difference arises from two factors in the Earth's orbital motion around the sun and its rotation about its own axis.
The Earth's orbit is not a circle but an ellipse, so that the sun's apparent motion is more rapid in January, when the Earth is nearer the sun, than in July, when it is farther away, the effect being enhanced by the fact that the Earth is moving more rapidly in January than in July, by Kepler's Law. The second factor in the equation of time is due to the Earth's axis being inclined by 23 1/2º to a line perpendicular to its orbit.
If we take a viewpoint from the Earth itself we see that, while hour angles must necessarily be measured around the celestial equator, the sun's apparent motion is around the ecliptic, and this motion is not uniform when projected on to the equator, being greater in summer and winter when the sun is moving parallel to the equator and is nearer to the poles than at the equinoxes when its motion is at 23 1/2º to the equator.
The two factors of the equation of time are roughly equal in magnitude, but the first goes through one cycle per year, the second two cycles. When plotted on a graph and aded together they give a curve with two major and two minor peaks.
A hand-operated mechanical instrument in the form of an elaborate volvelle by which the positions of the planets in the heavens can be found.
The dates, spring and autumn, at which day and night are of equal length, or, more precisely, the dates at which the sun's declination is zero.
On celestial or armillary spheres the equinoxes are the points at which the great circles of the ecliptic and equinoctial, or equator, intersect. On the present Gregorian calendar the spring equinox is on 20th or 21st March and the autumn on 22nd or 23rd September.
Equinoxes, Precession of
The positions of the equinoxes are determined by the position of the Earth's axis, and if this was constant in direction the equinoxes would remain fixed in position in relation to the fixed stars. Owing to a gyroscopic effect produced by the gravitational pulls of the moon and sun on the Earth's equatorial bulge, the Earth's axis, though remaining always at the same angle to the plane of its orbit, changes its direction, making one complete revolution in approximately 26,000 years. From the Earth, therefore, the equinox points in the heavens move slowly round the equinoctial or equatorial circle. The spring equinox in the early Christian era was in the constellation of Aries and was termed the 'First Point of Aries'. It has now moved back into the constellation of Pisces, a movement of nearly 30º of Right Ascension.
The clock mechanism that controls the swing of the pendulum or the movement of the balance wheel.
Escapement, Anchor, Inverted-anchor, Recoil
The recoil escapement most commonly found in pendulum clocks, first used in the last quarter of the 17th century. In an anchor escapement from a 19th-century bracket clock, the escape wheel rotates clockwise and gives impulse to the entry pallet. The pendulum is moving to the left. The tooth will leave the end of the entry pallet, and a tooth will drop on to the exit pallet. The pendulum will still be moving to the left and the exit pallet will drive the escape wheel backwards. This constitutes the recoil. When the pendulum's swing to the left is complete, the exit pallet will receive impulse from the escape wheel until the tooth leaves the end of this pallet; then the next tooth will drop on to the entry pallet while the pendulum is still swinging to the right. This will again reverse the motion of the escape wheel until the pendulum's swing to the right is completed. The cycle is then repeated.
The inverted-anchor escapement is a form found in American and German clocks; its action is as described above. The anchor is bent up from strip steel and is often mounted on an adjustable pillar on the dial plate of the clock. The pendulum crutch is riveted directly on the anchor.
A form of pin-pallet escapement invented by Louis-Gabriel Brocot, which, when fitted visibly in front of the dial of 19th-century French or American clocks, forms an interesting feature. The pallets are made from cylindrical pins of cornelian or steel. The diameter is slightly less than the distance apart of two escape-wheel teeth, the non-acting half of each pallet being cut away to give clearance. The locking is against the highest point of each semicircular pallet face. Impulse is given on the quarter-circular face from the highest point to the lower lip of the pallet. The escapement can be made dead-beat by cutting the locking faces of the escape wheel tangential to a circle drawn about the axis of the pallet staff, or the escapement may be made with slight recoil by cutting the escape-wheel teeth radial to its axis. The operation of the Brocot escapement is similar to the dead-beat.
Also known as 'remontoire escapement', and having many differing designs. These escapements achieve a uniform impulse by the use of the driving force to wind an auxiliary spring or weight during a detached part of the escapement's action; the energy stored in the auxiliary spring or weight is used to give impulse to the balance or pendulum while the clock train is locked, thereby providing a constant driving force to the escapement. The Secticon clock constant-force escapement was introduced in the 1960s. The driving motor roller winds the remontoire spring via the intermediate roller and the impulse arm, which is latched by the permanent magnet. On the anticlockwise vibration of balance, the impulse pin enters the fork of the impulse arm, which unlatches from the permanent magnet and gives impulse to the balance. At the end of the impulse arm's travel, the roller locking nib is advanced by pallet, enabling the action to continue. Constant-force escapements can give the highest performance, but cost of manufacture and difficulty in adjustment make this complicated escapement rather rare.
This escapement is sometimes found in high-quality French clocks; it enables a seconds hand to jump full seconds with a pendulum which beats half-seconds. This is usually achieved by a form of pinwheel escapement, having one pallet hinged to the anchor on to which the pins of the pinwheel drop. when the pendulum swings to the left, the weight at the outer end of the pivoted pallet lifts the coup-perdu pallet clear fo the exit pallet, enabling the pendulum to receive impulse on its swing to the right. The next pin drops on to the coup-perdu pallet, closing it on to the impulse pallet, thus continuing the cycle of operation.
This 16th-century escapement was an early attempt to improve on the verge. The friction caused by the necessity of coupling the two pallet staffs by gearing or cranks must reduce the performance relative to the verge.
Sometimes called the 'horizontal escapement', a form of which was patented by Thomas Tompion in 1695 and subsequently perfected by George Graham. It will be found in many platform escapements fitted to 19th-century, and later, carriage clocks, etc. This escapement, the earliest dead-beat escapement for watches, has a steel cylinder with resting surfaces inside and outside for the escape-wheel teeth. The lifting face of tooth B is giving impulse to the exit lip, the balance rotating anticlockwise until the tooth is released by the exit lip, allowing tooth C to drop on to the outside of the cylinder. The escapement remains under frictional lock until the balance, on its return swing, allows tooth C to move on to its impulse face against entry lip D, the balance thereby receiving impulse and tooth C eventually dropping into the frictional rest position inside the cylinder, until the balance again reverses and releases this tooth to continue the cycle. The apparent advantages of this simple dead-beat escapement are offset by the difficulty of retaining oil in its proper place, and by the large radius of the frictional rest from the center of rotation of the balance, which causes variations, the result of friction and rapid wear.
Escapement, Dead-beat, Half-beat, Vulliamy
The dead-beat escapement is attributed to George Graham and was introduced c. 1730. This escapement gives excellent results over long periods when fitted to well-made clocks. The locking faces form arcs of a circle centered on the axis of the pallet staff. The impulsing is similar in action to the anchor escapement, but between impulses the escape wheel remains stationary, locked on the faces on the pallet. This escapement is admirably suited to weight-driven longcase regulators. For spring-driven clocks, the half dead-beat escapement is more suited. In this escapement, the locking faces are formed to give an amount of recoil judged by the maker to counteract any changes in motive power which must occur in a spring-driven clock, as the resistance offered by the recoil counteracts the increase in pendulum arc which occurs when more motive power is available, and vice versa. Towards the end of the 18th century Benjamin Vulliamy introduced a variant of the dead-beat escapement in which the angle between the arms of the pallets is adjustable by a screw.
Escapement, Detached-lever, Straight-line lever, Right-angle lever
This escapement, which has finally ousted all others in mechanical balance-controlled timekeepers, was first invented in the mid 18th century, although its merits were not fully appreciated until the second quarter of the 19th century, when it began to be produced in large quantities.
The lever escapement may be classified in three main types, the action being identical in each case. The first type, made mostly by the English, has all the lift or impulse on the pallets, and the escape wheel has ratchet-type teeth. The second type has what is known as 'divided lift', i.e. some of the time pulse is on the escape-wheel teeth, which are of club shape, and the remainder of the lift is on the pallet stones. This form is fitted to most good-quality balance-control clocks today. The third type is the pin pallet, which has all the lift in the escape-wheel teeth. It is found in low-quality clocks and, although cheap to make, suffers from disadvantage of rapid wear, mainly because of poor oil retention.
The disposition of the parts allows further classification: English makers usually mounted the lever arm tangentially to the escape wheel, the balance, pivot and escape-wheel pivot holes making a right angle, while the Swiss planted the pivot holes of the balance, pallets and escape wheel along a straight line.
The escape wheel to pallets action is similar to the dead-beat escapement with one major exception, which is that the pallet stones are angled on their locking faces to give a small amount of recoil, used to draw the lever on to the banking pins. The forked end of the lever is about to receive the impulse pin. The balance is rotating anticlockwise, the impulse pin entering the lever fork, moving the lever to the left, and unlocking the escape wheel and receiving impulse. The escapement then re-locks on to the opposite pallet, and the balance continues its anticlockwise swing. The draw on the pallet stone moves the lever to contact the banking pin, making contact between the guard pin and the guard roller unlikely.
Frictional-rest escapements, such as the cylinder or duplex, suffer from the interference caused during the locking. In the lever escapement, interference is isolated from the balance during most of its swing, thus detaching the balance from its driving force, except during unlocking and impulse, and gaining the name 'detached-lever escapement'.
Escapement, Pivoted-detent, Spring-detent
During the 18th century, great efforts were made to develop a practical marine chronometer. John Arnold and Thomas Earnshaw, working on entirely different lines, developed the detent escapement, commonly called the 'chronometer escapement', Earnshaw's design being adopted by most makers since.
In the modern form of spring-detent chronometer escapement the discharge pallet is just beginning to move the detent to the left. The locking stone will release the tooth of the escape wheel, allowing an escape-wheel tooth to fall on impulse pallet. As the balance's clockwise swing continues, the discharge pallet releases the gold spring, allowing the detent to be returned by detent spring in readiness to receive the next escape-wheel tooth on completion of the impulse. On the return swing of the balance, the discharge pallet is allowed past the detent by the gold spring. This is the only escapement action in the anticlockwise half of its cycle. The action is known as 'single-beat'.
The chronometer escapement is even more detached than the lever; another great advantage is that the escape-wheel teeth and discharge pallet require no oil. Originally most makers mounted the detent on pivots, with a separate adjustable return spring. This system was used extensively by European makers during the 19th century, but the necessity of oiling the pivot holes was a disadvantage, and the detent and spring formed from one piece of steel is usually found in marine chronometers.
A type of frictional rest escapement, utilizing an escape wheel with two sets of teeth, one for locking and the other for impulsing. It is normally used only in watches, though occasionally occurring in 19th-century clocks for the Chinese market.
The story of the discovery of the isochronism of a pendulum by Galileo Galilei's observation of a swinging lamp in Pisa Cathedral is well known. Galileo believed that the pendulum took the same time to swing both wide arcs and small arcs, i.e. that it was isochronous, whereas, in fact, the shorter arcs take slightly less time than the wide swings. Vicenzio, Galileo's son, in 1641, drew a design of a form of duplex clock escapement, given to him by his father. The ratchet-shaped locking teeth, formed on the periphery of the escape wheel, are locked by the detent which is shown in the lifted position. The pendulum staff carries two arms; the lower arm is shown receiving impulse, while the upper is lifting the detent. No original examples of this escapement of this escapement are known to exist.
The very poor quality of oil led the brothers James and John Harrison to develop a clock escapement which would run without oil. The brass frame carrying the pallet arms is attached to the pendulum crutch. The pendulum is at the start of its swing to the right, and is receiving impulse from escape-wheel tooth on to the pallet. During the pendulum's swing to the right, the frame brings the pallet to intercept escape-wheel tooth, which receives a small amount of recoil, freeing the pallet from the tooth and allowing the spring to move the pallet clear of the escape wheel. The pendulum then moves to the left, receiving impulse from the escape wheel via the pallet and the tooth, until the pallet is again brought into contact with the next escape-wheel tooth, which under recoil releases the tooth from the pallet, enabling the cycle to continue.
Only recently have the merits of this interesting escapement begun to be appreciated. This early work by the Harrison brothers eventually led to John Harrison being awarded for the discovery of a method of finding a ship's longitude at sea to an accuracy of within half a degree of longitude, although the timekeeper which achieved this success was not fitted with the grasshopper escapement.
This escapement is a variant of the pinwheel escapement. In the usual French pinwheel escapement, the two pallet arms are of unequal length. Jean Andre Lepaute reasoned that the friction and impulse at the pallets must differ, and constructed an escape wheel with two sets of pins. The pallet arms are of equal length, one arm engaging pins on the front of the escape wheel, the other engaging pins on the back. This arrangement made impulse and frictional losses equal on both pallets.
Various arrangements have been tried to silence the tick of a clock, but none so successful as the magnetic escapement, in which energy is transferred from the movement to the pendulum by magnetic attraction. In a magnetic escapement invented by C.F. Clifford of Horstmann Clifford Magnetics Ltd, and similar to one marketed a few years ago, the escape wheel has a sine wave shaped rim, which runs between the poles of a magnet mounted in the upper part of the pendulum. The spokes and teeth on the escape wheel allow for the pendulum's free swing, impulse being given to the pendulum by the half-sine wave sections of the escape wheel rim.
The escapement has been used in an electrically maintained, transistor-switched, tuning-fork clock to drive the wheelwork from the vibrations of the tuning fork.
In this escapement the pallets are made from small-diameter pins which engage the club-shaped teeth of the escape wheel, all the lift and draw being formed in the escape-wheel teeth. This escapement is only fitted to low-quality clocks; it is often poorly made, and retention of oil on the working surfaces is another problem.
In this escapement, the escape wheel carries pins inserted into one or both sides of the rim and is usually made with the pallets dead-beat. The pallet arms are of different lengths when made to work with pins in only one side of the escape wheel. Although the layout is very different from the dead-beat escapement, the action is similar. The pinwheel escapement has the advantage that the pallets are always in tension, therefore wear in the pivot holes has less effect on timekeeping but, like the pin-pallet escapement, retention of oil on the pallets is poor, causing wear and inferior performance.
This escapement is commonly found in good-quality French clocks made in the second half of the 18th century, and sometimes in English turret clocks.
The development of escapements has aimed at reducing interference to the pendulum by the movement. Earlier this century, Sigmund Riefler produced a clock the pendulum of which is maintained by flexing the suspension spring. The upper chops of the suspension spring are carried by a block which is free to move on agate blocks along the line of flexure of the suspension springs. The block carries the anchor. The pallet pins engage two escape wheels for impulse and locking. This type of escapement provides outstandingly accurate timekeeping in mechanical clocks.
Escapement, Savage Two-pin
An early form of lever escapement, designed by George Savage of Huddersfield, Yorks., in which the impulse pin is replaced by two pins engaging a wide lever notch. This allows the pins, which only perform an unlocking function, to roll in the lever notch, thus reducing friction. Impulse is given to the balance through the guard pin working in a square notch in the roller.
Clocks for use in bedrooms are required to produce as little noise as possible. To this end various escapements fitted with resilient pallets have been made. In a verge bracket clock fitted with gut pallets, the escape wheel has triangular-shaped teeth which engage short pieces of gut stretched between spring pallet arms carried by the pallet staff. The escapement action is identical to the verge.
Escapement, Tic-tac or Drum
A form of anchor escapement in which the anchor only spans one to three teeth of the escape wheel. It is fitted to French drum clocks with short pendulums swinging through a large arc. Sometimes one pallet is formed to give frictional rest only, all the lift being on the other pallet.
The earliest turret-clock escapements were of the verge-and-foliot type. There was the occasional variant like the one used by Richard of Wallingford, comprising two wheels on the same axis between which the balance was pivoted. A curved bar at the bottom of the balance engaged alternately the spokes of the two wheels. After Christiaan Huygens developed the pendulum in 1650 a number of clocks were built with crown wheel and large-arc bob.
The improved timekeeping of the recoil anchor escapement in 1671 brought a demand for conversion from verge-and-foliot, and the dead-beat escapement also found wide application in turret clocks for many years. It appears in various forms apart from the normal anchor type. In some the pallets are adjustable, either by sliding through the bow of the anchor or by a two-part bow, the relative positions of which can be adjusted by a screw. In one variant the teeth protrude axially from a disc rather than radially from the wheel's periphery.
The pinwheel was also widely used. Evidently it had a good reputation, as a few recoil escapements were converted to pinwheel by the expedient of inserting pins in the rim of the wheel, leaving the teeth in position. Some very large pinwheels were made, and sometimes the pins fell on pallets fixed directly to the pendulum and not through a crutch.
A major improvement was the gravity escapement. The commonest version is the double three-legged. As the pendulum swings, it engages the beat pin on one of the gravity arms, moving it outwards. This unlocks the arbor which rotates 1/6 turn, locking on the other gravity arm. During the 1/6 turn, one of the three lifting pins has lifted the gravity arm a little as the pendulum moves. On the return stroke, therefore, the gravity arm moves under its own weight a greater distance than it moved out. The difference of the two movements represents the energy delivered to the pendulum, i.e. the impulse. The importance of the gravity escapement is that the impulse delivered to the pendulum is constant and quite independent of the load on the clock resulting from wind and weather. The fly on the 'scape arbor is essential to provide enough damping to prevent tripping. Other versions of this escapement have appeared - double four-legged and single three-legged. In one version, the impulse takes place on one side only. Commonly 15 legs are used.
The first escapement known, used in most clocks until the introduction of the recoil anchor escapement in the third quarter of the 17th century. By the end of the 18th century the verge was rarely made, and many earlier verge-escapement clocks were modernized by fitting an anchor escapement. A tooth of the escape wheel gives impulse to the pallet flag. On completion of impulse, a tooth drops on to the pallet, receiving recoil as the pendulum forward swing continues. When the motion of the pendulum is reversed, the pallet receives impulse until the tooth escapes, and the next tooth drops on to the other pallet, which receives recoil until the pendulum reverse swing is complete, the cycle then continuing.
The verge escapement has a undeservedly bad name for timekeeping. A clock in good condition, well maintained, will average one or two mintues of error per week over long periods.
The trim around a keyhole.
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Invented in 1906 by an electrical engineer named T.B. Powers of the United States. The Eureka clock was the first electrically maintained balance-wheel clock. Convinced that he had devised a perfect timekeeper, Powers called his clock 'Eureka' that in Greek meant I found it. Manufacture did not start until 1909.
The earliest models were enclosed under a glass dome where the large compensated balance could be seen swinging. Later models were fitted in a variety of cases. Many of the Eureka clocks have only 'Eureka Electric Clock - 1000 days' on the dial to indicate that the clock is electric. Timekeeping did not prove as good as Powers had hoped,
though the clock made with the balance and mechanism visible had a powerful novelty appeal. The arm of the balance is a soft iron core with two soft iron arms which form a U-shaped magnet when current passes through its solenoid winding. A silver pin on the balance arm touches a fixed silver contact at the correct point for impulsing the balance,
the balance arm being attracted to a fixed soft iron armature; the resulting swing operates a cam on the balance arbor to move the clock train through a pawl and ratchet wheel. About 10,000 of these clocks were made before manufacture ceased in 1914.
An eyeglass is an optical instrument required by the clockmaker to magnify the image of an object and bring it closer to the eye. Eyeglasses may consist of a round, tapered tube fitted with a lens, usually single but occasionally multiple, and shaped to be held in the eye socket; or they may be held to the eye by a band worn round the head. Eyeglasses are also made in hinged frames which can be fitted to spectacles as required, and old illustrations show eyeglasses held in bench stands over the work. Lenses of different powers of focal length are available, and two-, three-, or four-in. eyeglasses are used when the work is to be viewed at those distances. A double eye glass will allow the object to be seen c. 1/4in. from the lens. Eyeglasses with a tube are internally finished in matt black to prevent disturbing reflections, and have a ventilation hole in the side to prevent condensation on the lens.
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