ELECTROMAGNETIC AFFECT OF COXVECTIOX-CURREXTS 253 



the iield will be more irregular, and so make accurate measurements 

 more difficult. 



In the present apparatus a uniform field is secured by using two 

 vertical disks rotating about horizontal axes in the same line; the needle 

 sy.-tcin is placed between the disks, opposite their centres. The disk? 

 are in the meridian; they are gilded on the faces turned towards the 

 needle. Between the disks are placed two glass condenser-plates gilded 

 on the surfaces near the disk; and between these glasses is the needle. 

 The whole apparatus is symmetrical about the lower needle of the 

 astatic system. 



Each disk is surrounded by a gilded hard rubber guard-plate in order 

 to keep the density of the charge uniform at the edges. The guard- 

 plates are provided with adjusting-screws to enable them to be put 

 accurately in the plane of the disks; and the glass plates in turn have 

 adjusting-screws for securing parallelism with the guard-plates. The 

 glass was carefully chosen as being nearly plane. Disks, glass plates, 

 and guard-plates all have radial scratches, to prevent conduction-cur- 

 rents from circulating around the coatings. 



In the periphery of the disk are set eight brass studs which pene- 

 trate radially for about 5 centim., then turning off at a right angle run 

 parallel to the axis until they come out on the surface of the disks. 

 They there make contact with the gold foil. Metal brushes set in the 

 guard-plate bear on these studs, and in this way the disks are electrified. 



The figure (PI. V, Fig. 1) gives a vertical projection of the entire 

 disk-apparatus : D D are the disks ; G G G G the guard-rings ; Y Y Y Y 

 the condenser-plates ; R R R R hard rubber rings fitting on the should- 

 ers A A; X X X X bearing-boxes for the axle; P P P P supporting- 

 standards ; E E metal bases sliding in the bed B B, and held in any 

 position by screws Z ; F F the bases carrying the glass plates, sliding in 

 the same way as the others. S S S 8 are the adjusting-screws for the 

 guard-plates, and 1 1 for the glass plates. L L L L are collars for catch- 

 ing the oil from the bearings; C C, C' C' are speed-counters, C C gear 

 with the axle, and C' C' with C C in the manner shown; each has 200 

 teeth, and speed-reading is taken every 40,000 revolutions. 



The needle system is enclosed in the brass tube T, ending in the 

 larger cylindrical box in which are the mirror and upper needle. This 

 is closed in by the conical mouth-piece Q, across the opening of which 

 is ] daced a wire grating. The mirror is shown at M, the upper needle 

 at y and the lower at N. The system is hung by a fibre-suspension 

 about 30 <?m. in length, protected by a glass tube. The needle- 



