324 BELL SYSTEM TECHNICAL JOURNAL 



ballistic aspect; however, the concept is much broader than this since it is 

 not at all dependent upon a corpuscular concept of the electron. As a result 

 of this property of the total current, the current to any electrode within a 

 vacuum tube does not necessarily bear any relationship to the number of 

 electrons which enter or leave it. Obviously then, currents can exist in the 

 grid circuit of a three-element tube even though none of the electrons are 

 actually intercepted by the grid. This current may have any phase rela- 

 tionship to an impressed voltage on the grid so that the grid may draw power 

 from the external circuit, or it may deliver power to the external circuit, 

 all without actually intercepting any electronic current. The grid-current 

 component resulting from the electronic flow between cathode and plate 

 may equally well bear a quadrature relationship to the impressed voltage, 

 in which case it will either increase or decrease the apparent interelectrode 

 capacitance. If these effects seem queer it is because one is still confusing 

 the electronic component with the total current. 



A second basic concept once stated becomes self-evident. This is to the 

 effect that the only one thing which we can do to an electron is to change its 

 velocity, that is, if we are to confine ourselves to the classical concept of 

 an electron. We can change its longitudinal velocity, that is, alter its speed 

 but not its direction other than possibly to reverse it, or we can introduce a 

 transverse component to its velocity, that is, alter its direction as well as its 

 speed. Thought of in this light all electronic devices in which a control is 

 exercised over an electron stream are velocity-modulated devices. It might 

 be argued that one could equally well say that all we can do is to change the 

 electron's acceleration {derivative of velocity) or its position {integral of velocity). 

 The singling out of velocity is in a sense arbitrary. It does, however, have 

 some very interesting ramifications. 



I might digress for a moment to elaborate on this idea. Since some of 

 the newer devices have been labeled velocity-modulation tubes, there is a 

 perfectly understandable tendency on the part of the uninitiated to assume 

 that these tubes differ from earlier known devices, such as, for example, the 

 space-charge-control tubes, the Barkhausen tube or the magnetron in the 

 fact that they employ velocity modulation. The real difference lies else- 

 where as we shall see in a few moments. At the same time that these newer 

 devices were introduced, there was introduced a new way of looking at 

 something which is very old in the art. This newer viewpoint, to my way of 

 thinking, constitutes a far greater fundamental contribution than do the 

 specific devices which have received so much attention. The pioneers in this 

 new approach: Heil and Heil, Bruche and Recknagel, the Varian Brothers, 

 Hahn and Metcalf, to mention a few, and the many other workers who lost 

 in the race to publish their independent contributions in this field — all of 

 these people deserve the greatest of praise for their stimulating contributions 



