18 BELL SYSTEM TECHNICAL JOURNAL 



tween the deflector plates. The beam of electrons produces slowly 

 moving positive ions and electrons in the gas along its path between 

 the plates. When a voltage is applied to the plates, the positive ions 

 travel from the beam toward the negative plate and an equal number 

 of the electrons travel toward the positive plate. The space charge 

 set up by the electrons and ions produces an electric field opposing 

 that created by the applied voltage. The greatest space charge occurs 

 near the negative plate where the sluggish positive ions flow. The 

 space midway between the plates remains nearly field free and there 

 is little deflection of the beam until the voltage is greater than that 

 at which all of the ions produced are drawn to the plate. Calculation 

 agrees with observation that this voltage is 2 to 3 volts on either side 

 of zero. 



Having now described the operation and structure of some of the 

 oscillographs, I should like to say something more about their uses. 



The cathode ray oscillograph is essentially a curve tracer that plots 

 out in rectangular coordinates the relation between two quantities 

 represented by the fields between the deflector plates. Often one of 

 these quantities is time, as in the ordinary moving mirror oscillograph, 

 while the other is some electrical quantity. We then say that we plot 

 a wave shape. We must then have a way of making the spot move at 

 a uniform rate. One of the simplest and most reliable ways of pro- 

 ducing a linear time axis,^^ at least for low voltage tubes, employs two 

 thermionic tubes to control the charge in a condenser as shown in 

 Fig. 22. One tube, a simple two-electrode tube Ti, limits the charging 



Fig. 22 — Thermionic tube "sweep circuit." 



current of the condenser C so that the voltage across the condenser 

 rises linearly with time according to the equation V = Cit. The 

 second tube To is filled with gas and has the property of passing cur- 

 rent only when the voltage across it reaches a certain value, which 

 in turn is controlled by the grid potential. When the condenser volt- 

 21 A. L. Samuel, Rev. Sci. Inst., 2, p. 532, 1931. 



