196 BELL SYSTEM TECHNICAL JOURNAL 



These last two formulae are for the first focus. Focii will also be obtained 

 for values of H equal to nH where n is an integer and equal to the number of 

 electronic revolutions. Actually as the field is increased beyond that neces- 

 sary for the first focus the beam does not get very badly out of focus because 

 the radius of curvature of the spiral path is small and for still higher fields the 

 beam remains in approximate focus for all values of H. 



In applications where the beam is rotated by means of a rotating mag- 

 netic field this electrostatic field is made to turn by separating the anode 

 structure into four or six elements (or groups thereof) and applying either 

 two- or three-phase alternating potentials to them. 



Magketic Field Supply 



The stator of a two-pole pol^'phase alterating-current motor furnishes an 

 excellent magnetic field for use with these tubes. The tube is inserted in 

 place of the armature and when the polyphase currents are applied the 

 beams are formed and rotate at the cyclic frequency. For applications where 

 the beams are not rotated continuously, a two-phase stator may be used in 

 which the currents through the two windings are adjusted to be proportional 

 to the sine and cosine of the desired direction angle of the beam. Per- 

 manent magnets of the horseshoe design have also been found to be suitable. 



The power consumed by a stator depends on its size and the strength of the 

 field it produces and on the cyclic frequency if it is used to rotate the beam. 

 At low frequencies, e.g., 20 or 60 cycles, the power consumed is primarily 

 that due to the copper loss. At higher frequencies the losses in the core 

 material become important. For some of the smaller tubes operating at a 

 low frequency, the power consum.ed by the stator is less than three watts. 

 This stator has the regular motor windings which do not completely fill 

 the slots. 



Since a pol>T)hase source of power is not always readily available, it is 

 sometimes advantageous to split single-phase power in the stator itself to 

 produce the rotating field. This may be done by inserting a condenser in 

 series with each winding so that the current through one phase winding lags 

 by 45° and that through the other leads by an equal angle. Polyphase po- 

 tentials for producing a rotating electrostatic field in the tube may then be 

 taken from the windings of the stator if desired. 



Tube Design 



The particular design of tube depends on its application. The simple 

 design shown in Fig. 1 has been found adequate for som.e purposes but more 

 elaborate designs which increase the versatility of the tube are also needed. 



Figure 4 shows a tube with 30 anodes that incorporates various auxiliary 

 elements. This tube is 2.25 inches in diameter. Figure 5 shows the internal 



