Pendulum clock

A pendulum clock uses a pendulum as its time base. From their invention, in 1656, until the 1930s, clocks using pendulum movements were the most accurate. Because of their need to be stationary and immovable while operating, pendulum clocks cannot operate in vehicles; the motion and accelerations of the vehicle will affect the motion and pace of the pendulum, causing inaccuracies. See marine chronometer for a discussion of the problems of navigational clocks.

Pendulum wall clock

Pendulum Wall Clock

History

The pendulum clock was invented by Christiaan Huygens in 1656, based on the pendulum introduced by Galileo Galilei.

Pendulum clocks remained the mechanism of choice for accurate timekeeping for centuries, ending with the Shortt free pendulum observatory clocks invented in 1921 and the pendulum clock of Edward Hall that marked the end of the pendulum era as the most reliable time standard.

Pendulum clocks remain popular for domestic use.

Mechanism

Pendulum clocks typically have four parts:

• a mass at the end of a pendulum rod
• an escapement system that passes energy to the pendulum to preserve oscillations and which releases the gear train in a step-by-step fashion
• a gear train that slows the rapid escapement rotation to a rate matching the motor characteristics
• an indicator system that records how often the escapement has rotated and therefore how much time has passed

Gravity-swing pendulum

The pendulum swings with a designed period that varies with the square root of its effective length.

Thermal compensation

To keep time accurately, pendulums are usually made to not vary in length as the temperature changes. Owing to the expansion of metal, the length of a simple pendulum will vary with temperature, slowing the clock as the temperature rises. Early high-precision clocks used the liquid metal mercury to lift a portion of the pendulum mass in compensation for the increased length of the suspension. John Harrison invented the gridiron pendulum, which uses a sliding "banjo" of solid metals with differing thermal expansion rates such as brass or zinc and steel to achieve a zero-expansion pendulum while avoiding the use of toxic mercury.

By the end of the nineteenth century, materials were available that had a very low inherent change of length with temperature and these were used to make a simple pendulum rod. These included Invar, a nickel/iron alloy; and fused silica, a glass. The latter is still used for pendulums in gravimeters.

Atmospheric drag

The viscosity of the air through which the pendulum swings will vary with atomspheric pressure, humidity, and temperature. This drag also requires power that could otherwise be applied to extending the time between windings. Pendulums are sometimes polished and streamlined to reduce the effects of air drag (which is where most of the driving power goes) on the clock's accuracy. In the late 19th century and early 20th century, pendulums for clocks in astronomical observatories were often operated in a chamber that had been pumped to a low pressure to reduce drag and make the pendulum's operation even more accurate.

Local gravity

As a pendulum clock is necessarily stationary, the clock will be adjusted for local gravity. Since an increased gravity will increase the pendulum speed a pendulum clock not adjusted after movement may be used as a gravimeter when small time differences between this and other types of clocks (or a clock at a fixed location) are measured. Other modern forms of gravimeters measure gravity by accurately timing the free fall of a proof mass.

Torsion-spring pendulum

This pendulum is a wheel-like mass (most often four spheres on cross spokes) suspended from a vertical strip (ribbon) of spring steel. Rotation of the mass winds and unwinds the suspension spring, with the energy impulse applied to the top of the spring. As the period of a cycle is quite slow compared to the gravity swing pendulum, it is possible to make clocks that need to be wound only every 30 days, or even only once a year. A clock requiring only annual winding is sometimes called a "400-Day clock", "perpetual clock" or "anniversary clock", the latter sometimes given as a wedding memorialisation gift. Schatz and Kundo, both German firms, were once the main manufacturers of this type of clock. This type is independent of the local force of gravity and is less affected by temperature changes than is an uncompensated pendulum.

Escapement

(see a more detailed description here)

The escapement drives the pendulum, usually from a gear train, and is the part that ticks. Most escapements have a locking state and a drive state. In the locking state, nothing moves. The motion of the pendulum switches the escapement to drive, and the escapement then pushes on the pendulum for some part of the pendulum's cycle. A notable but rare exception is Harrison's grasshopper escapement. In precision clocks, the escapement is often driven directly by a small weight or spring that is re-set at frequent intervals by an independent mechanism called a remontoire. This frees the escapement from the effects of variations in the gear train. In the late 19th century, electromechanical escapements were developed. In these, a mechanical switch or a phototube turned an electromagnet on for a brief section of the pendulum's swing. These were used on some of the most precise clocks known. They were usually employed with vacuum pendulums on astronomical clocks. The pulse of electricity that drove the pendulum would also drive a plunger to move the gear train.

In the 20th century, W.H. Shortt invented a free pendulum clock with an accuracy of one-hundredth of a second per day. In this system, the timekeeping pendulum does no work and is kept swinging by a push from a weighted arm (gravity arm) that is lowered onto the pendulum by another (slave) clock just before it is needed. The gravity arm then pushes on the free pendulum, which releases it to drop out of engagement at a time that is set entirely by the free pendulum. Once the gravity arm is released, it trips a mechanism to reset itself ready for release by the slave clock. The whole cycle is kept synchronised by a small blade spring on the pendulum of the slave clock. The slave clock is set to run slightly slow, and the reset circuit for the gravity arm activates a pivoted arm that just engages with the tip of the blade spring. If the slave clock has lost too much time, its blade spring pushes against the arm and this accelerates the pendulum. The amount of this gain is such that the blade spring doesn't engage on the next cycle but does on the next again. This form of clock became the standard for use in observatories from the mid-1920s until superseded by quartz technology.

Time Indication

The indicating system usually consists of two hands moving round a circular dial that carries twelve large markers for the hours and sixty markers for the minutes. Many clocks have a small third hand indicating seconds on a subsidiary dial. The gear train is usually arranged so that one of the arbors turns once in an hour. This is used to drive the minute hand through a slipping clutch that allows the position of the hand to be adjusted by being pushed round the shaft. The hour hand is usually driven not from the main train but from the minute hand through a small set of gears.

See also

• Pendulum
• Escapement
• Steam clock
• Odd sympathy

External links

• The Invention of Clocks