358 Mr Southertis, Ewperimental Investigation as to Dependence 



few lines offeree from D to pass between the two, and some of these 

 will terminate on the edges of B. Those falling on the upper and 

 lower edges in the figure give rise to forces tending to deflect the 

 beam. If everything is symmetrical these forces will be equal 

 above and below, and therefore the beam will not actually be 

 moved. But if B is slightly higher than its central positioo, the 

 lower gap will be wider than the upper one and more lines of 

 force will therefore fall on the lower edge of B than on the 

 upper edge. These will tend to bring B back again to its central 

 position. The beam is always in stable equilibrium under the 

 action of the field. The application of the field, however, affects 

 to a slight degree the sensitiveness of the apparatus as a balance, 

 and it is therefore necessary to have some means of determining 

 the sensitiveness while the field is on. This is provided by the 

 arrangement shewn in the upper part of the figure. It is an 

 adaptation of a method employed in a balance designed by 

 Dr Hicks and used by the writer in the research previously alluded 

 to. A small and feeble magnet is attached to the beam as shewn 

 by the dotted lines at F. This can be attracted or repelled by a 

 small current passing round the coil 0. The deflection produced 

 by a given current can be compared once for all with that pro- 

 duced by moving a rider a certain distance along the bar H. By 

 observing the deflection produced by this current during an actual 

 experiment with the plates charged, the sensitiveness can be 

 determined, and the value of any deflection produced by the 

 charges expressed in mgms. of weight. The current is not of 

 course allowed to pass during actual observations. The current 

 required to produce a measurable deflection of the beam, though 

 small, is vastly in excess of any possible leakage current between 

 the field plates in the balance, and therefore it is impossible that 

 these should act directly on the magnet and give rise to any 

 difficulty by causing deflections. The cell, resistance and key, 

 N, P and M in fig. 3, are used for supplying the current. The 

 rider is not moved during a series of observations. It is used 

 before observations are commenced in order to level the beam. 

 By suitably levelling the beam the electrostatic deflection 6 may 

 be varied at will, both in magnitude and direction. Many values 

 of this deflection have been used in the experiments. 



The deflections are observed by telescope and scale, and a 

 double suspension mirror, the position of which is indicated by the 

 circle K. The field plates D, E are insulated by sulphur plugs 

 and have various adjustments not shewn on the figure. Other 

 details, such as knife edge adjustments, etc., are also left out of 

 the figure to avoid complication. The front and back of the case 

 are of thick magnalium with plate glass windows. The latter 

 have been covered inside with tinfoil in the later experiments. 



