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POPULAR SCIENCE MONTHLY. 



form was immediately developed into what was called a " focus 

 tube/' in which a similar concave focusing cathode was employed, 

 and a platinum plate inclined at forty-five degrees to the axis of 

 the cathode rays was inserted between the two electrodes of the 

 tube to receive the impact of the cathode rays as in No. (4). A 

 plate so placed is called an anticathode. This idea was carried 

 still further to produce the double-focus tube shown in No. (5), 

 which is especially suited to oscillatory discharges from the elec- 

 trodes, and therefore adapted to use with alternating current 

 apparatus, especially Tesla coils. In this form of tube the anti- 



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I.. 



2 

 Tig. 3. 



-Typical Forms of Crookes's Tubes. 



cathode consists of a wedge-shaped piece of platinum midway 

 between the ends of the tube. If this platinum terminal be con- 

 nected with the positive pole and both end electrodes with the 

 negative pole, this tube is very efficient with a Ruhmkorff coil 

 giving unidirectional discharges. For any tube there is a critical 

 degree of vacuum as well as electric potential, with which it is 

 most efficient. Tubes can be made suitable for a coil giving a 

 spark of not more than an inch, but they are not very energetic. 

 Since the vacuum rises with continued use of a tube, some forms 

 e. g.. No, (5) have a small side tube communicating with the main 

 bulb and containing caustic potash or other substance which 

 volatilizes on being heated, so that its vapor will reduce the 



