ACCURATE TIMING

Hawera Star, Volume LIII, 17 June 1933, Page 11

ACCURATE TIMING

5000th Part of Second

HPALKING pictures are being improved every -*• day, but most of the research devoted to this science is being conducted upon photoelectric cells, writes Professor A. M. Xtow, in the *‘Sunday Chronicle ” A talking film sometimes looks like an ordinary strip of film upon which are situated a number of lines of different intensity', or depth. When' these lines are passed in front of a lens the light is made dim or bright in accordance with the markings on the film. These changes in light can be converted into changes in electric current, which is then amplified and connected to an ordinary loud speaker. In practice a photo-electric cell consists of small plates of metals which produce very minute electric currents when light falls upon them. The rare metals, such as calcium, caesium and potassium, all possess this effect. The metal is sealed inside a small glass tube so that the air cannot reach its surface. By choosing metals of different types it is possible to work these cells with invisible light, and it is on this principle that various burglar alarms have been designed. High-speed motor racing is now'timed in the sam ! e fashion by causing the passing motor-car to interrupt a beam of light from the lantern to the photo-electric cell. The cell switches off the current on a clock device as the ear stops the light from reaching its surface. Professor Low adds: ‘'Timekeeping is quite an art. Bullets can be timed without difficulty', and a degree of accuracy within one fivethousandth part of a second is quite an everyday' 5 affair.”