200 BELL SYSTEM TECHNICAL JOURNAL 



of the center of one of the screen elements or one of the suppressor groups and 

 a minimum current will be received by an anode back of the junction of two 

 such elements or groups. If two-phase supply is used (4 elements or groups) 

 the ratio of maximum to minimum anode current will be 0.707 and for three- 

 phase supply this ratio will be 0.866. There will be 4 or 6 maxima, respec- 

 tively, around the tube. This variation may be effectively eliminated by 

 varying the individual anode load impedances or in other ways. 



The anode characteristics are similar to those of a pentode if suppressor 

 grids are used and to that of a tetrode if these grids are not used. 



There is still another method of effectively eliminating one beam. This 

 consists in using an odd number of anodes so that when one beam is focused 

 on an anode the opposite one falls on the screen in between two anode posi- 

 tions. With this type of tube the effective rotational frequency is twice 

 the cyclic frequency of the rotating field, that is, all of the anodes are con- 

 tacted twice (once for each beam) per revolution of the field. 



Applications 



The many possible combinations of the tube elements just described per- 

 mit a variety of applications. One of the simplest and most obvious is that 

 of an electronic commutator which has the advantages over the correspond- 

 ing mechanical device of speed and freedom from contact trouble. There is, 

 however, a practical limitation to the speed of this electronic commutator 

 that is set primarily by the alternating-current losses in the stator. This 

 is estimated to be in the neighborhood of 10,000 cycles per second for ordi- 

 nary stator and tube designs. The highest cyclic speed for a stator that 

 has been used to date was 600 cycles per second which with utilization of 

 both beams gave an effective cyclic frequency of 1200 cps. 



One of the earliest systems of multiplex telegraphy was based on time 

 division using mechanical rotating commutators. A small portion of the 

 time of one cycle of the moving brush was allotted to each channel. The 

 usefulness of this system is limited because of the faults of the mechanical 

 commutators. The substitution of these electronic commutators eliminates 

 these difficulties and puts the time division system on a more practical basis. 

 It has an advantage over the frequency division multiplex system (carrier 

 system) in that the elaborate filters of the latter are not required. 



A 30-channeI multiplex system for signaling using two of the 30 anode 

 tubes described above has been successfully tested over short distances in the 

 metropolitan area in New York City. The tube at the transmitter had all 

 of the anodes tied together and the signal from them was sent over the line. 

 The 30 input channels terminated on the suppressor grids of this tube. At 

 the receiver, the input was fed to the negative grid surrounding the cathode 

 and each of the anodes was connected in scries with a small neon lamp for 



