CLOCK

machine in which a device that performs regular movements in equal intervals of time is linked to a counting mechanism that records the number of movements. All clocks, of whatever form, are made on this principle. The origin of the all-mechanical escapement clock is unknown; the first such devices may have been invented and used in monasteries to alert the sacristan to toll a bell that called the monks to prayers. The first mechanical clocks to which clear references exist were large, weight-driven machines, fitted into towers and known as turret clocks. These early devices did not strike the hour, nor did they have hands or a dial. The first public clock that struck the hours was made and erected in Milan in 1335. The oldest surviving clock in England is that at Salisbury Cathedral, which dates from 1386. A clock erected at Rouen, Fr., in 1389 is still extant, and one built for Wells Cathedral in England is preserved in the Science Museum, London. The Salisbury clock strikes the hours, and those of Rouen and Wells also have mechanisms for ringing chimes at the quarter hour. Other English clocks of somewhat later date were large, iron-framed structures driven by falling weights attached to a cord wrapped around a drum and regulated by a mechanism known as a verge (or crown wheel) escapement. Their errors probably were as large as a half hour per day. The first domestic clocks were smaller versions of these large public clocks. They appeared late in the 14th century, and few examples have survived; most of them, extremely austere in design, had no cases or means of protection from dust. These clocks stood on a pedestal with an aperture to accommodate the weights. About 1500 Peter Henlein, a German locksmith, began to make small clocks driven by a spring. These were the first portable timepieces, representing one of the great strides in horology. The dials of these clocks, placed on the top and possessing an hour hand only (minute hands did not appear until 1670), were exposed to the air; there was no form of cover such as a glass until the 17th century (see also watch). From the end of the 16th century, clocks were made in the upright form. They were similar to the domestic weight-driven clocks but were not portable. During the early part of the 17th century, the mechanism was enclosed and the cases were made of brass. About 1582 Galileo noticed the characteristic timekeeping property of the pendulum. The Dutch astronomer and physicist Christiaan Huygens was responsible for the application of the pendulum as a time controller in clocks from 1656 onward. Huygens' invention brought about a great increase in the importance and extent of clockmaking. Clocks, weight driven, with short pendulums, were encased in wood and made to hang on the wall. In 1670 the long, or seconds, pendulum was introduced by William Clement, an English clockmaker. The next step was to enclose the pendulum and weights, and the long-case, or grandfather, clock was born. The pendulum is a reliable time measurer because, for small arcs, the time required for a complete swing (period) depends only on the length of the pendulum and is almost independent of the extent of the arc. The length of a pendulum with a period of one second is about 39 inches (990 millimetres), and an increase in length of 0.001 in. (0.025 mm) will make the clock lose about one second per day. Altering the length of a pendulum is, therefore, a sensitive means of regulation. The alteration is usually carried out by allowing the bob to rest upon a nut that can be screwed up or down the pendulum rod. Any expansion or contraction of the rod caused by changes of temperature will affect the timekeeping of a pendulum; e.g., a pendulum clock with a steel rod will lose one second a day for a rise in temperature of 4 F (2.2 C). For accurate timekeeping, the length of the pendulum must be kept as nearly constant as possible. This may be done in several ways, some of which use the differing coefficients of expansion (the amount of expansion per degree change in temperature) of different metals to obtain a cancelling-out effect. In one popular compensation method, the bob consists of a glass or metal jar containing a suitable amount of mercury. The gridiron pendulum employs rods of brass and steel, while in the zinc-iron tube, the pendulum rod is made up of concentric tubes of zinc and iron. A less expensive method, however, is to make the pendulum rod from a special alloy called Invar. This material has such a small coefficient of expansion that small changes of temperature have a negligible effect. In a pendulum clock an escape wheel is allowed to rotate through the pitch of one tooth for each swing of the pendulum, and to transmit an impulse to the pendulum to keep it swinging. An ideal escapement would perform both functions without interfering with the free swing. The double three-legged gravity escapement was invented by Edmund Beckett, afterward Lord Grimthorpe, and used by him for the great clock at Westminster, now generally known as Big Ben, which was installed in 1859. It has since become standard for all really accurate tower clocks.