156 Dr. W. F. G. Swann on the Magnetic Field produced 



with silk, and shellacked. The coils were connected in such 

 a way that on rotating them together the E.M.F/s due to the 

 earth's magnetic field acted in opposite directions round the 

 circuit, so that the effect of this field was eliminated, and 

 what is more important, any effects due to its fluctuations 

 were automatically eliminated. The three vertical plates 

 shown in the figure formed two condensers ; the central 

 plate, which was insulated in sulphur, could be charged, 

 while the other two were fixed to a brass plate which could 

 be earthed : thus any magnetic fluxes due to the earth's 

 motion through space were additive in their effects on the 

 galvanometer. The coils were partially embedded in rect- 

 angular blocks of paraffin wax, which also served to hold 

 them in position, and they were arranged symmetrically with 

 respect to the axis of rotation, so thnt the system was balanced 

 when rotating. The galvanometer used was of the Broca 

 type, and contained two coils each of resistance ?>2 ohms ; 

 these coils were arranged in series, and the point of junction 

 was earthed. By earthing this point, and in virtue of the 

 symmetry of the arrangement, various small leakage effects 

 which caused trouble in the preliminary experiments were 

 successfully overcome ; for example, gradual leakage from 

 the charged plate, and consequent alteration of the induced 

 charge on the surfaces of the coils, resulted in electricity 

 coming up from earth, and some of this came through 

 the galvanometer, but in virtue of the symmetry of the above 

 arrangement, equal quantities came through the two galva- 

 nometer coils, and consequently their effects on the galvano- 

 meter-needle cancelled. 



The apparatus was mounted on a base as shown in the 

 figure, in such a manner that it could be turned about, and 

 the base could be clamped to the table in either a horizontal 

 or a vertical plane by means of a nut. The rotating coils 

 were connected by two pieces of copper foil cemented 

 to the axle, which consisted of an ebonite rod with a brass 

 core, and the system could be rotated by a motor placed 

 2 metres away. 



The central plate, which was connected to the inner coating 

 of a leyden-jar, was charged from a Wimshurst machine, 

 one pole of which was earthed, and the potential of the plate 

 was measured with a Kelvin electrostatic voltmeter. 



It was necessary to carefully earth one pole of the wims- 

 hurst, as otherwise currents were produced through the 

 tables &c, which were sufficient to cause slight kicks in the 

 galvanometer when the condenser was charged or discharged. 

 Kicks of this kind were not of vital importance, since they 



