of the Weight of a Body on its state of Electrification. 353 



Suppose a delicately but stably-balanced earthed conductor A 

 to be placed between two fixed charged conductors B, G, at 

 potentials + F, , - Fg respectively. Two electrostatic fields F^ , F^ 

 will be set up, and these will be quite independent of each other, 

 provided all lines of force from B to G are intercepted by the 

 conductor A, which is at zero potential, or by some other earthed 

 conductor. Charges will be induced on the ends of A, which will 

 now be subject to electrostatic forces which will in general deflect 

 it to a new position inclined at an angle, say 0, from its original 

 equilibrium position. If now the fields F^, F^ be reversed each 

 independently of the other, so that we have B at potential — Fj 

 and G at potential + V^, there will be no change in this deflection 

 6 caused by electrostatic forces, for these are independent of the 

 direction of the fields. But now let us suppose that a gravitation 

 effect exists such as to cause the end of A, which is positively 

 charged, to be a little heavier than the negatively charged end. 

 This will give a deflection + h which must be added to 6, thus 

 with one direction of field we shall have a total deflection 6 -{-h, 

 and with the direction of field reversed we shall have deflection 

 6 — ^. The difference between the two observed deflections will 

 thus be 2S, the electrostatic part of the deflection being eliminated. 

 In observing deflections practically two methods may be adopted. 

 We may observe the zero position under no field, then put on the 

 field in a given direction and observe ^ + S, discharge and re- 

 determine zero, then charge in the opposite direction, and observe 

 6 — h and take the difference. Or we may disregard the zero and 

 merely, having the field in a given direction, observe position of 

 A, then reverse the field and note the deflection 28 which results. 

 Both these methods have been used. 



It will be seen that there is no theoretical necessity for making 

 the fields or the conductors symmetrical or the potentials Fj, V^ 

 numerically equal to each other. In practice, however, it is 

 desirable to make the apparatus and fields as symmetrical as 

 possible, though it is quite possible to obtain results with wide 

 differences between the potentials Fj, Fg. With symmetrical 

 apparatus the deflection 6, due to electrostatic forces, will be 

 small and any deflection due to a gravitation etfect will be more 

 readily observed. 



The form of apparatus indicated in fig. 2 is therefore adopted. 

 The beam AA with its curved end pieces is supported by a steel 

 knife edge and grooved plate by means of which it is electrically 

 connected to earth. The pierced guard cylinder D, and metallic 

 case E, are also earthed. The field plates B, G are insulated 

 and connected through a double reversing key to the sources of 

 potential, which are separate for the two plates. Some of the 



