354 Mr Southerns, Bocperiinental Investigation as to Dependence 



lines of force from these plates fall on the guard cylinder and on 

 the case. The apparatus can be adjusted so that the forces acting 

 on AA are very nearly horizontal, so that they produce only a 

 very small deflection from the uncharged position. This deflection 

 is usually made small for practical convenience, but the resulting 

 value of S is the same for large as for small deflections. The 

 stability of the arrangement and method of determining sensitive- 

 ness while under the electric field will be referred to later. It 

 may be mentioned here that variations of weight corresponding to 

 5Wo ^^ Towo '^goQ- i^re easily detected in practice. 



This method of arranging the apparatus and of applying and 

 reversing the fields eliminates several errors which might be 

 expected to cause difficulty in an experiment of this nature. As 

 shewn above, the effect of electrostatic forces (which do not depend 



Fig. 2. 



on the direction of the field) is eliminated on reversal. The same 

 also applies to any heating effect due to currents which pass while 

 charging and reversing, or to leakage currents, and also to electro- 

 magnetic effects, none of which depend on the direction of the 

 currents or fields. A point which has often been mentioned to the 

 writer is the difficulty of eliminating stray lines of force. These 

 are really unsymmetrical portions of the field which go to make 

 up the deflection 6 as described above and are included in the 

 preceding discussion. Experiment shews that great departures 



