THE luminosity produced by an electric current passing through a gas at low pressure varies greatly in character, not only when we alter the nature of the discharge, as, for example, when we pass from the arc to the spark, but also in many cases at different points of the same discharge. The luminosity may be of one colour at one place and of a very different colour at another, and spectroscopic examination shows that the spectrum of the same gas often varies considerably as we proceed along the line of discharge. As recent experiments have thrown a considerable amount of light on the processes going on in the different kinds of electrical discharge and at different parts of the same discharge, the study of the connection between the changes in the electrical effects and the changes in the spectra might be expected to throw some light on the very interesting question of the genesis of spectra. Many important points can very conveniently be studied by the aid of Wehnelt's method of producing the current. In this method the kathode is a strip of platinum or a piece of platinum wire on which either a little lime or barium oxide has been deposited. This when heated to redness emits large supplies of corpuscles, and by altering the temperature of the platinum very large variations in the current passing through the tube and in the potential difference between the electrodes can be obtained. In our experiments the current varied from a small fraction of a milliampere to several amperes, and the potential difference from a few volts to several hundred.
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Physical Review (1913)