of the Weight of a Body on its state of Electrification. 355 



from symmetry do not vitiate the result. Thus the field plates 

 may be moved, or we may use either of the plates alone, the other 

 being earthed, or we may adjust the beam in such a way that d 

 is large or small and in either direction from the zero. In all 

 these cases the result is unaffected *. In most of the experiments 

 the balance was screened from any possible external electrostatic 

 effects, this however made no difference to the result — stray lines 

 coming on to the beam from the external electrical apparatus gave 

 rise to no discrepancy, these will be dealt with more fully when 

 the experiments themselves are described. The effects do not 

 depend on strict accuracy being attained in the magnitude of the 

 potentials employed. A variation of eight volts out of a thousand 

 purposely introduced produced no effect on the results. This 

 shews that the results are not due to some variation of potentials, 

 such as might be caused by contact difference. 



Reference will be made later to some effects observed with 

 electrometers which seem to have a bearing on the subject, but it 

 should be borne in mind that the present instrument is not an 

 electrometer, indeed it is the exact opposite, that is to say it is 

 designed so that electric charges produce the least possible de- 

 flection instead of the greatest as with the electrometer. 



(2) Description of apparatus. 



The apparatus was for the most partf constructed by the writer 

 (who was aided by a Royal Society grant) at the Technical College, 

 Huddersfield, and afterwards set up in the Cavendish Laboratory, 

 Cambridge. The general plan of the arrangements is shewn in 

 fig. 3. 



In the figure, A represents a table standing on rubber blocks 

 and supporting the main instrument or balance B which is enclosed 

 in a wooden box. C is a table on which are placed an observing 

 telescope D, a scale E, with its sliding support and lamp F, a 

 double pendulum electroscope G, a multi-cellular voltmeter H, 

 keys J, K, L, M, Daniell cell N, and resistance box P, the uses of 

 which will be described later. Another table R carries a cabinet 

 of 1000 small secondary cells in two separate trays of 500 each, 

 which are connected to the key L by the leads T. The diagram 

 is approximately to scale, the distance from telescope to balance 

 being about two and a half metres. The dotted lines represent 

 connecting wires which are insulated on paraffin wax columns. 



* Irregular fluctuations occur in the magnitude of the result, but they are 

 extremely small for an experiment of this kind, 

 t Knife edges by Oertling. 



