move forward a fixed amount at each swing. This is the source of the "ticking" sound of an operating pendulum clock. The pendulum, a weight on a rod, which is the timekeeping element of the clock An indicator or dial that records how often the escapement has rotated and therefore how much time has passed, usually a traditional clock face with rotating hands. Additional functions in clocks besides basic timekeeping are called complications. More elaborate pendulum clocks may include these complications: Striking train: strikes a bell or gong on every hour, with the number of strikes equal to the number of the hour. Some clocks will also signal the half hour with a single strike. More elaborate types, technically called chiming clocks, strike on the quarter hours, and may play melodies or Cathedral chimes, usually Westminster quarters. Calendar dials: show the day, date, and sometimes month. Moon phase dial: Shows the phase of the moon, usually with a painted picture of the moon on a rotating disk. Equation of time dial: this rare complication was used in early days to set the clock by the passage of the sun overhead at noon. It displays the difference between the time indicated by the clock and the time indicated by the position of the sun, which varies by as much as ±16 minutes during the year. Repeater attachment: repeats the hour chimes when triggered by hand. This rare complication was used before artificial lighting to check what time it was at night. In electromechanical pendulum clocks such as used in mechanical Master clocks the power source is replaced by an electrically powered solenoid that provides the impulses to the pendulum by magnetic force, and the escapement is replaced by a switch or photodetector that senses when the pendulum is in the right position to receive the impulse. These should not be confused with more recent quartz pendulum clocks in which an electronic quartz clock module swings a pendulum. These are not true pendulum clocks because the timekeeping is controlled by a quartz crystal in the module, and the swinging pendulum is merely a decorative simulation. Gravity-swing pendulum The pendulum in most clocks (see diagram) consists of a wood or metal rod (a) with a metal weight called the bob (b) on the end. The bob is traditionally lens-shaped to reduce air drag. Wooden rods were often used in quality clocks because wood