Geoscience Reference
In-Depth Information
13
Radioactivity: invisible
geochronometers
INVISIBLE RAYS
While most people can name the inventor of the television - the
Scottish scientist John Logie Baird (1888-1946) - few of us can remem-
ber who invented the cathode ray tube, a fundamental part of the
television without which it just would not work. For the first cathode
ray tube, we can thank Sir William Crookes (1832-1919), an affluent
chemistwho had established his ownprivate laboratory inLondon. The
kind now used in televisions is a variant designed by Karl Ferdinand
Braun (1850-1918), the German physicist whose name is now onmany
domestic electrical appliances. While working with a Crookes Tube in
his laboratory at the University of W
¨
rzburg in Germany in 1895, the
physicist WilhelmR ¨ ntgen (1845-1923) made a remarkable discovery.
When he turned the cathode tube on he discovered that barium plati-
nocyanide sitting on a shelf on the other side of the roomstarted to glow
slightly. He then removed the chemical to the next room, and it still
glowed. The explanation suddenly occurred to him: while the tube was
producing cathode rays it was also producing another invisible ray that
had the ability to pass through thin sheets of metal, which the cathode
rays could not penetrate. As he had no clue what the rays were or what
formed them he simply called them 'X-rays'. He then experimented
with their properties and produced an image of his wife's hand by
placing it on a photographic plate and exposing it to the X-rays. While
the traces of her flesh could be detected, her bones as well as some rings
were easily seen. Immediately a medical use was found for these new
invisible rays. When others in the early twentieth century referred to
themasR¨ ntgen Rays his name became universally adopted.
Shortly after R
¨
ntgen's ground-breaking discovery a different
source of invisible radiation was discovered. These rays were different
