340 SECTIONAL TRANSACTIONS.— A. 



Mr. A. L. Whiteley. — Photo-electric control in industry (11.45). 



A comparison is made between the photo-electric (emissive) cell and other 

 available types of photo-sensitive device, showing the advantages of the 

 forementioned as regards sensitivity, speed of response and stability of 

 characteristics. 



The standard types of photo-electric relay are briefly described. The 

 usual form of relay functions solely to open or close an electric circuit 

 according to the level of illumination on its photo-electric cell. 



Applications to machine tools, textile, printing and allied industries, 

 control of street lighting, etc., are illustrated. 



Applications relying upon the high-speed response characteristics of the 

 photo-electric cell are shown to be among the most important uses of the 

 device. Relatively simple photo-electric control can respond to a light 

 change lasting less than yoW second. 



A further useful property of the caesium type of photo-electric cell is its 

 rapid increase in output when the temperature of the light source is raised. 

 For instance : if the temperature of the radiating body is raised from 1700° F. 

 to 1800° F. the output of the photo-electric cell is doubled. This is the 

 basis of operation of the photo-electric pyrometer, which has a number of 

 advantages over other temperature-measuring instruments. 



The alleged disadvantages of vacuum tube type apparatus in industry 

 are dealt with ; experiences with this class of apparatus in service are briefly 

 described. 



Prof. J. A. Carroll. — Modern eclipse observing (12.10). 



To obtain the information sought in modern eclipse observing, full 

 perfection of the optical parts of the instruments used is required. Modern 

 optical manufacture is able to provide parts capable of yielding the full 

 theoretical resolving powers, but only under conditions of good thermal 

 equilibrium. In the laboratory this is easily attained, but in the field large 

 regular and irregular diurnal changes of temperature are experienced. 



The expedition to Omsk attempted for the first time full thermostatic 

 control of the observing instruments. Interferometer observations necessi- 

 tate thermal control, but practically all instruments can benefit materially 

 from it. The arrangements used are described, and the success attained 

 demonstrated. 



A brief description of the programme of observations, the particular 

 instruments used and the results obtained on this occasion are given, and 

 also some account of Dr. R. L. Waterfield's observations of the Corona 

 at Chios. 



Dr. A. D. Thackeray. — The spectrum of the extreme limb of the sun (12.30). 



The few seconds at the beginning and end of a total solar eclipse offer 

 a unique opportunity of studying the spectrum of the extreme limb of the 

 sun and watching the transition from the Fraunhofer absorption spectrum 

 to the bright line spectrum of the chromosphere. Enhanced lines which 

 are characteristic of high temperature in the laboratory are stronger in the 

 chromosphere than in the Fraunhofer spectrum. This seemed to point 

 to the curious conclusion that the upper layers of the sun were hotter than 

 the lower layers until ionisation theory showed that the lower pressure 

 in the upper atmosphere would produce much the same effect as high 

 temperature. 



