LOW VOLTAGE CATHODE RAY OSCILLOGRAPH 149 



10~* second. When the beam moves it has to build up the ionization 

 as it goes along, and we should expect that when deflected very 

 rapidly it might no longer be focused, due to lack of positive ions in 

 its path. A test was made of this by applying a high frequency 

 potential on the deflector plates so that the spot described an elliptic 

 pattern. At a frequency of 10* cycles per second the line was still 

 sharp, but at 10* cycles there was a noticeable widening of the line 

 which is probably to be ascribed to imperfect focusing at this high 

 speed . 



In these experiments the evidence all points to the view that the 

 focusing of the electrons is caused by an excess of positive charge in 

 the beam itself, produced by ionizing collisions of the electrons with 

 the gas molecules. Further confirmation is found in the fact that a 

 focus is much more readily obtained in the heavier gases having slow 

 molecules, such as nitrogen, argon or mercury vapor, than in hydrogen 

 and helium where the mean velocity of the molecules is greater. The 

 tubes are therefore filled with argon, the heaviest available permanent 

 gas which does not attack the electrodes. The best pressure for the 

 length of tube adopted and for the current which can be obtained 

 in the beam is 5 to 10 microns, and this leaves considerable latitude 

 for the adjustment of the electron current to get a sharp focus. 



Examples of the Use of the Tube 



Because of the small amount of auxiliary apparatus required with 

 this form of Braun tube it has proved to be a very convenient labora- 

 tory instrument. It has found application in studying the behavior 

 of vacuum tubes and amplifier and oscillator circuits, of gas discharge 

 tubes, of relays, and of numerous other kinds of apparatus, both at 

 low and at high frequencies. Some reproductions of photographs of 

 various types of curves are given below to illustrate the kind of results 

 which are possible with this oscillograph. 



Fig. 5 shows the hysteresis curve of a sample of iron wire. The 

 wire was placed in a small solenoid with one end toward the side of 

 the tube. The magnetizing current passed through a resistance, 

 the voltage drop of which was applied to one pair of deflector plates 

 so as to give a deflection proportional to the magnetizing field. The 

 stray magnetic field from the iron itself produced the deflection 

 proportional to the induction. Alternating current was used, and 

 the exposure was 20 seconds with lens opening / 6.3 and speed roll 

 film. 



In Figs. 6a and 6b are shown the current-voltage relations of an 



