We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.
Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.
SOURCES OF SOUND CURRENT
517
The two elements of the photo-cell are connected across a source of extremely steady, ripple-less direct current, drawn either from a dry battery or from an exceptionally well-filtered rectifier supply. One such circuit is shown in Figure 120. (16) Negatrons emitted by the cathode when light strikes it are attracted to the positive anode. Thi> action constitutes a flow of current across the cell. Because the emission depends entirely upon the light reaching the cathode, the cell acts as a highly sensitive valve, permitting current to flow through it (anode voltage being constant) in proportion to the amount of light to which it is exposed. Since the response of the cathode is substantially instantaneous, the cell finds no difficulty in dealing with light fluctuations that correspond to even the highest frequencies of audible sound.
IZ TO I
ZooV.dc — o o
P/ZI.
Fig. 120. — Photo-cell coupling.
TO
FADER.
S£C.
(Transformer coupling)
The arrangement of Figure 120 results in a flow of pulsating direct current through the primary winding of the 12-to-l ratio transformer. The amperage flowing (the micro-amperage, to be more exact) will fluctuate in strength as many times per second as the light entering the cell changes, and will be proportionate in its strength to tlie strength of that light.
Thus the opaque and transparent areas of the sound track are converted, through the motion of the film and the operation of the photo-electric cell, into corresponding pulsations of direct current.