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NEGATRONS PROM PHOTO-ELECTRIC ELEMENTS 453
ductor of heat. The outer surface of this substance is coated with emitting metal, usually thorium. The heater current flows in one end of the hair-pin wire, around the hair-pin bend and out at the other end of the wire. It is insulated from and never touches the thorium, which serves as an emitter only because it becomes hot enough to give off a relatively large quantity of negatrons.
Cold Emitters
(7) Although tubes based upon that principle are seldom used in theatre equipment it may be of theoretical interest here to include a word about a tube that operates with a cold cathode or emitter. There is a tube used in some radios and a few theatres which consists essentially of two pieces of metal inside a vacuum. One of these metals has a very much larger surface than the other. Both metals, even though cold, emit some negatrons, but since the emission is proportional to the surface, the vastly greater emission from the metal with the larger surface can be made to constitute a current flowing across the tube to the metal with the smaller surface.
How emitted negatrons are made to flow across a vacuum to another element within the glass, and thus to constitute a current, is explained on Page 499. Here we are interested only in methods by which negatrons may be obtained.
Negatrons from Photo-electric Substances
(8) Certain metals which, like all metals, at all times emit a few negatrons, possess the property of increasing their emission when exposed to light.
(9) Caesium and potassium are two metals in which this property is most strongly apparent, and these metals are used as emitting surfaces in photo-electric cells.
This action is not nearly as well understood as emission depending upon heat. (10) The cause of increased