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chemical mixtures is shown.
The manner in which this system operates is as follows : In reverting to the first classification of photo-tube uses (A) in which the light ray penetrates the subject. We have in this system the light source (A) which directs the light ray through the liquid solution (B) which upon increasing in density due to the addition of chemicals, permits less light to pass to photo-tube (C) that due to its directly proportionate output causes a correspondingly less current to flow to amplifier (D) which in turn permits relay (E) to work, passing power to valve (F) stopping the flow of chemicals to the beaker. This is representative of the daily use in chemical and pharmatical laboratories of the phototube.
In the next example which is representative of classification (B) we have a circuit shown in Fig. 5 where a reflecting mirror is used to direct more or less light to the phototube. In this particular case a thermocouple is suspended in a hot bath or place where the accurate control of heat is necessitated to a microscopically small degree as in retorts, blast furnaces, plating baths, etc. The output of the thermo-couple is connected to a sensitive galvanometer which carries a small reflecting mirror (M). The light ray (L2) is then directed to the stationary mirror (Ml) thence, to photo-tube. From the photo-tube the current goes to the amplifier circuit shown, and then to relay (R) from the relay the controlling current permits the addition of more or less heat to the subject under control. The light source (L) is concentrated by lens (LI) upon the galvanometer mirror.
Other uses of the reflecting system with the same circuit, with the exception of the galvanometer is that of grading and sorting of various ob
Figure 4
jects and color comparison by placing the subject to be scanned. Where the rays from the light source reflect from the subject to the photo-tube, the relay either stops the operation of the machinery or sounds a warning, notifying the attendant in charge that something is amiss. In this manner exceedingly accurate comparisons of various subjects may be had.
In the last classification (C) where the direct light rays from a source are permitted to impinge upon a photo-tube, we have a means of measuring light intensity and (depending upon the photo-tube composition) the various light spectrum that are present. In this manner some of the main uses are in stella photometry, light photometery and spectral photometery. One of the latest uses is that of an exposure meter mounted in a camera to give an instantaneous means of measuring the intensity of illumination on the set as picked up in the
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