Scientific Instruments

Abstract

Pendulums are used in various scientific instruments, either to make measurements, or to demonstrate scientific principles. Four of these uses are described in this chapter. These are Kater’s pendulum, Newton’s cradle, Foucault’s pendulum, and the Charpy impact testing machine. If the effective length, l, of a pendulum and the pendulum period, P, are known then the value of the acceleration due to gravity, g, can be calculated. The practical difficulty is the measurement of the effective length of a real pendulum. At the suggestion of F W Bessel in 1817 Captain Henry Kater of the British Army developed a reversible pendulum, known as Kater’s pendulum, whose effective length can be accurately determined. Kater’s pendulum, and subsequent refinements, were used for the determination of g until they were superseded by free-fall gravimeters in the 1950s. In its usual form, a Newton’s cradle, consists of five identical pendulums each with a hardened steel ball suspended by a pair of strings with the balls just in contact. If the ball at one end of the row is pulled back and released the impulse as it hits the next ball is transmitted along the row, and the ball at the other end flies off with the remaining balls remaining almost stationary. The Newton’s cradle has a long history. In 1662 Huygens pointed out that an explanation of its behaviour required conservation of both momentum and kinetic energy, although he did not use the latter term. Newton’s cradle is still used to demonstrate this point. The term is used because the foundations of mechanics, including conservation of momentum and kinetic energy were established by Isaac Newton in his Principia of 1687. Newton’s cradle is also an amusing toy. The Foucault pendulum was used by Léon Foucault to demonstrate the Earth’s rotation in 1851. Apart from their use in clocks the Foucault pendulum is probably the best known use of pendulums. There are examples in many museums around the World. A Foucault pendulum usually consists of a spherical steel bob suspended from a long steel wire, with a pointer mounted below the bob so that its motion can be easily followed. A Foucault pendulum can swing freely in any direction, and is a close approximation to a simple string pendulum. In a Charpy impact testing machine a swinging pendulum is used to fracture notched steel test pieces. The energy absorbed in breaking a notched test piece is a useful measure of a steel’s fracture toughness. Interest in the effects of impacts on metals dates back to the early nineteenth century. Impact testing of metals developed from the observation that metals are often more brittle under an impact than when loaded slowly. The earliest description of an impact test appears to be that given by Tredgold in 1822. He states: ‘The best and most certain test of the quality of a piece of cast iron, is to try it with a hammer; if the blows of the hammer make a slight impression, denoting some degree of malleability, the iron is of good quality, providing it be uniform; if fragments off, and no sensible indentation be made, the iron will be hard and brittle.’ The first use of a pendulum impact testing machine was by S Bent Russell in 1898, and by the beginning of the twentieth century the use of pendulum impact testing machines was well established. The pendulum impact testing machine developed by Georges Charpy in 1904 is the basis of the Charpy impact testing machines that are now extensively used world wide. Methods of test have been standardised, although there are some variations. Despites its importance the Charpy impact test is not usually mentioned by writers on pendulums.