A B C of television; or, Seeing by radio; a complete and comprehensive treatise dealing with the theory, construction and operation of telephotographic and television transmitters and receivers; written especially for home experimenters, radio fans and students.. (1929)

Record Details:

Title:
A B C of television; or, Seeing by radio; a complete and comprehensive treatise dealing with the theory, construction and operation of telephotographic and television transmitters and receivers; written especially for home experimenters, radio fans and students..
Date:
1929
Publisher:
New York, The Norman W, Henley publishing co.
Volume:
A B C of television; or, Seeing by radio; a complete and comprehensive treatise dealing with the theory, construction and operation of telephotographic and television transmitters and receivers; written especially for home experimenters, radio fans and students..
Leaf #:
000177
Language:
English
Contributor:
Prelinger Library
Sponsor:
MSN
Coordinator:
Media History Digital Library
Date Added:
July 20, 2022
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146 A. B. C. OF TELEVISION or inertia, but also adds considerable to the life of the cell through prolonging any chemical action that may take place. The sensitiveness of a cell is measured as the ratio between its resistance in the dark and its resistance when exposed to a source of illumination. An average cell may have a ratio be tween 2:1 and 3:1. However, well designed cells may have a ratio of sensitiveness as high as between 4:1 and 5:1. A cell having a resistance of 250,000 ohms can be reduced to 60,000 ohms when exposed to the light of even a low-power lamp. At this point it might be well to mention that it is necessary TERMINALS •—« INSULATING FORM FIG. 48.—Bidwell selenium cell is constructed by winding two wires upon an insulated form. Selenium is placed between the wires. to use some care in exposing selenium cells to continuous illu mination. Under such conditions cells are permanently affected and their subsequent action becomes sluggish, the ratio between light and dark resistance falling as much as 30%. A cell re sponding to such conditions is said to be fatigued, and once fatigued it becomes practically valueless for either television or phototelegraphic purposes. In the design of selenium cells that are to meet certain requirements of operation we must first consider the degree of sensitivity required and the factor of lag. We have noted heretofore that the specific electrical resistance of selenium is very high in the untreated state. Bidwell, an authority on selenium cells and their phenomena, estimated that the resist ance of selenium might be placed as high as 2500 megohms per