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ELECTRICAL APPARA'i 97
metals are somewhat more porous than \ special,
unusually dense alloy has to be used to preserve tinvacuum. (The outside air would seep in. though verj slowly, through a thin envelope of ordinary steel.
beads are fused into the metal envelope i insu
lators through which are led the wires that connect to the internal elements. The metal envelo] I the anode.
Except for serving as an electro-magnetic and electro static shield (warding off stray magnetic or static fields I it takes no part in the tube action.
(102) The photoelectric cell is a two-element tube, or diode, now invariably gas-filled, in which emission is not secured by heating the cathode, but by illuminating it using a beam of light from an external source. F I reasons still not clearly understood, thin film -tain
metals of the "alkali" or "alkali-earth" families, particu larly potassium and caesium, become moderately efficient in emission when illuminated, although they do not emit appreciably in the dark or when illuminated with light of the wrong color. A curved copper surface is coated with a thin film of photo-sensitive metal, which serves as the cathode of the cell. When light falls upon it, it emits electrons. An anode consisting of a thin copper rod is sealed into the same envelope. When positively charged from an external source, the anode attracts the electrons. The result is a device through which current will flow in exact proportion to the amount of illumination to which it is exposed. At any given illumination, the current in a gas-filled tube is greater, for reasons already explained. than it would be in a vacuum tube of the same construe tion and working at the same voltage. 1 fence photocells now are gas-filled. The gas used in modern ph<>t<>(v1U is generally argon.