vacuum tube (vakuumtub)

A sealed gla tube employed to examine the effects of a discharge of electricity through air or other gas rarefied or exhausted to the required degree. IMG:content-1671.png:[merged small][ocr errors] The most striking phenomenon is the magnificent colored light with which the tube is filled and the stratification of the light about the tube, the color of the light being different at the positive and negative electrodes, and va rying with the gas through which the discharge is pa ed. Thus, in common air it is purple or red at the positive end blue or violet at the negative; in hydrogen, it is greenish-blue; in carbonic oxid, bright-green, turning to yellow at the positive, and to blue at the negative. These tubes were first made by Gei ler of Bonn, and hence have been called Gei ler's tubes. A Crookes's tube is a form of vacuum-tube used by Mr. William Crookes in his investigation of what he has called radiant matter (which see, under radiant ). The exhaustion of these tubes is carried to about one millionth of an atmosphere.