288 PROCEEDINGS OF THE AMERICAN ACADEMY. 



or mica diaphragms, which serve to limit the cathode stream to a small 

 pencil, were replaced by glass diaphragms, sealed into the tubes and 

 having the shape of a truncated cone. The apertures of the dia- 

 phragms are from .5 to .7 mm. in diameter, and give a very sharply 

 defined luminous spot of about 1 to 1.5 mm. diameter on the fluorescent 

 screen. Figure 12 shows the general form of the tubes and the glass 

 diaphragms. 



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Figure 12. Braun Tube and Deflecting Coil. 



Braun tubes making use of electrostatic deflection of the cathode 

 beam were also used in parts of the experiment. They are essentially 

 the same as those just described with the addition of the two electro- 

 static deflecting plates. These plates are about 8 by 2.5 cm. in 

 dimensions, and are sealed inside the tubes about 2.5 cm. apart. 



Several Braun tubes have been made at the laboratory, and it was 

 found important that the distance between the cathode and the anode 

 should not be less than about 15 cm. A very sensitive fluorescent 

 screen for visual observation is made by dusting very finely powdered 

 willemite over a piece of mica or glass freshly painted with a thin coat 

 of water glass, although according to Zenneck a zinc sulphide screen is 

 to be preferred for photography and was used in this research. 



In order that the cathode beam may be homogeneous, that is, have 

 at all times the same velocity and consequently the same deflection 

 under a certain force, it is necessary that the potential of the source 

 causing the discharge be very constant. This restriction, of course, 

 excludes induction coils and transformers, and practically limits the 

 desirable sources to either the large static machine or the high potential 

 battery. 



If a static machine is used, one of many plates is necessary in order 

 to give sufficient intensity to the cathode beam, and if possible the 

 plates should have no sectors or carriers. 



The ideal source of current for a Braun tube is the high-voltage 

 secondary battery, capable of giving a potential of from 10,000 to 

 20,000 volts. In order to control the current, should the vacuum 

 drop, the current from such a plant should pass through two or three 



