THE ULTRASTABLE SYSTEM 8/8 



definition will be shown in each of the three requirements. 

 Firstly, the whole system, now of eight variables (four of the 

 magnet- deviations and four of the uniselector-positions), is abso- 

 lute, because the values of the eight variables are sufficient to 

 determine its behaviour. Secondly, the variables may be divided 

 into main variables (the four magnet-deviations), and step-func- 

 tions (the variables controlled by the uniselector-positions). 

 Thirdly, as the uniselectors provide an almost endless supply of 

 step-function values (though not all different) we do not have to 

 consider the possibility that the supply of step-function changes 

 will come to an end. In addition, the critical states (those 

 magnet-deviations at which the relay closes) are all sited at about 

 a 45° deviation ; so in the phase-space of the main variables they 

 form a 4 cube ' around the origin. 



It should be noticed that if only one, two, or three of the 

 units are used, the resulting system is still ultrastable. It will 

 have one, two, or three main variables respectively, but the critical 

 states will be unaltered in position. 



Time 



Figure 8/8/4 : Behaviour of one unit fed back into itself through a uniselector. 

 The upper line records the position of the magnet, whose side-to-side 

 movements are recorded as up and down. The lower line (U) shows 

 a cross-stroke whenever the uniselector moves to a new position. The 

 first movement at each D was forced by the operator, who pushed the 

 magnet to one side to make it demonstrate the response. 



Its ultrastability can now be demonstrated. First, for sim- 

 plicity, is shown a single unit arranged to feed back into itself 

 through a single uniselector coil such as A, D being shorted out. 

 In such a case the occurrence of the first negative setting on 

 the uniselector will give stability. Figure 8/8/4 shows a typical 

 tracing. At first the step-functions gave a stable field to the 

 single main variable, and the downward part of D l9 caused by 

 the operator deflecting the magnet, is promptly corrected by the 

 system, the magnet returning to its central position. At R v 



97 



