no THE PATH OF SCIENCE 



When it is positively charged, the anode will attract the 

 electrons, and the current will flo^w W^hen it is negatively 

 cliarged, it will repel the electrons and no current will flow, 

 so that an alternating current applied to a valve tube will be 

 transformed into a pulsating current in one direction, the 

 pulsations in the opposite direction being suppressed by the 

 tube. Then if a grid is inserted in the tube, the flow of the 

 electrons can be controlled by the charge on the giid. If a 

 signal current is applied to the grid, the flo^v of electrons 

 through the tube will follow the signal current, and in this 

 way a small signal current can be enormously amplified by 

 means of a tube. 



In the so-called photo tubes, electrons are emitted when 

 light falls on the cathode, and thus a beam of light can con- 

 trol an electric current which will follow the variations in 

 the light. By means of a photo tube, we can transform light 

 signals into electric signals and then by means of amplifying 

 tubes increase the electric currents so that they can perform 

 all sorts of operations. In this way, the reproduction of 

 sound can be accomplished. The sound waves can be used 

 in a microphone to control an electric current that can make 

 a lamp glow, and the variation of the intensity of the light 

 will therefore correspond to the sound. This variation can 

 be recorded photographically on a film. Light passing 

 through the record can be used to produce a current in a 

 photo tube, and this current can be amplified to operate a 

 loud speaker, by which the sound can be reproduced. 



W. D. Coolidge, working in the research laboratory of the 

 General Electric Company, improved the x-ray tube very 

 much by using a hot cathode, from which electrons ^vere 

 emitted even in a vacuum too high for the passage of elec- 

 tricity from a cold cathode, and by using a heavy tungsten 

 target that could withstand the powerful beam instead of the 

 thin platinum target used previously. At the same time, the 

 machines used to generate the electricity were gixatly im- 

 proved, and in this way x-ray sources of great intensity were 

 made available. 



