Positive rays and isotopes

The discovery of positive rays, canal rays, by Goldstein in 1886 is another important result of the studies of electrical condition in gases. The device is shown in Fig. 19.2. 

The cathode C has a hole, a canal, drilled through it. In addition to the usual discharge between A and C, a luminous stream emerges from the canal to the left of the cathode. This ray is positively charged and, reasonably enough, is called a positive ray. The systematic study of positive rays was long delayed, but it was determined at an early date that their characteristics depended on the kind of residual gas in the tube. In contrast, the cathode ray did not depend on the residual gas. 

Thomson was engaged in the measurement of the e/m of positive rays by the same general method as he used f or the electron when, in 1913, he discovered that neon consisted of two different kinds of atoms one having a mass of 20, the other having a mass of 22. These different atoms of the same element are called isotopes, meaning same place (that is, in the periodic table). Since this discovery that an element may contain atoms of different mass, the isotopic constitution of all the elements has been determined. Moreover, as is well known, in recent years many artificial isotopes have been synthesized by the high-energy techniques of physics. 

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