66i 



and the lid of c ; in consequence, however, of the unequal distance 

 from a to those diiFerent parts of c, the potential gradient pei- unit 

 of lengtii or the electric force, as also tlie density of the lines of 

 force, or the value of the tensions directed along the lines of 

 force in the space round a would be of very nnequal value ; how 

 great, however, the variation in ditFerent directions might be, yet 

 there would be complete symmetry in the course of the lines of 

 force with respect to the vertical plane in which the needle itself 

 is situated. The presence of l>, however, disturbs this symmetry in 

 the following way : 



1. The lines of force starting from a in the direction of /; will 

 no longer end on c, but on b; besides, on account of their diminished 

 length, therefore on account of the increased electric force, they 

 become there denser than before. 



2. There will be inflection of lines of force; some lines of force, 

 viz. those which, when not subjected to the influence of h, would 

 run beside b from a to c, w'\\\ pass into lines of force from a to h 

 under the influence of b. 



This disturbance caused by b will give rise to the formation of a 

 resulting electrostatic couple, acting on those halves of the side faces 

 of a, which are directed to the side of the acute angle between a 

 and b, so that consequently a is deflected to the side of b, and the 

 angle between a and b will become such that the formed torsion 

 couple of the suspension wire will be in equilibrium with the 

 electrostatic directive couple. 



In order to make a return to the untwisted position, c is charged 

 to a negative potential, which brings about the desired change; for 



'I. then (he density of the lines of force between a and c will 

 increase, which causes a slighter variation of lines of force on those 

 halves of the sides of a which are directed to the side of the acute 

 angle between a and b ; 



2. some lines of force between a and b will deflect and become 

 lines of force between a and c. 



In case of a sufticient negative i)otential of c the above mentioned 

 electrostatic resulting couple will be annihilated through this change. 

 The course of the lines of force has now become more symmetrical 

 (of course not quite), while a returns to the untwisted position. 



In this way e. g. a state of charge is realized for a z= -]- 20 V., 

 6 = v., c =1 — 3 V. 



For the sake of simplicity a whole number was taken for the 

 potential of c, the consequence of which is, that in the final state 

 the needle is only approximately in the untwisted position. 



