SM200, SM246     Discharge Tubes     circa 1930   Electricity

  In 1857 the German scientist Geissler constructed tubes in which he produced a vacuum so that he could observe the effect of electrical discharges inside.   But the vacuum produced was never perfect (a residual pressure of about 1mm of mercury remained) and the tubes produced a discharge whose colour was characteristic of the gas contained inside.

  The Englishman, Crookes, following the work of Geissler, increased the vacuum inside the tubes (leaving a residual pressure of only about 0.2mm of mercury).   He applied a high voltage and observed that the glass itself became luminous, this time with a colour characteristic of certain minerals contained in the glass mixture.   He showed this phosphorescence to be the result of invisible cathode rays discovered in 1859.   Later he identified this phenomenon to be the result of a flux of electrons, the same effect as that which causes the television tube to become luminous.

   Sir William Crookes 1832 to 1919