the Conductivity of Liquids. 329 



Accordingly the experiments were continued with the very 

 powerful and delicate apparatus shown in Plate VIII. One 

 litre of liquid is put into the vessel A, which consists of a 

 glas3 shade, the open end of which has been roughened with 

 emery, and a strip of strong brown paper attached to it with 

 paste. Two vertical strips of ebonite are fastened to the 

 paper ring with shellac, and bound round with cotton, and the 

 whole well varnished with shellac varnish. These strips are 

 fastened by ivory pins to the boxwood beam B ; and to the 

 centre of this beam the torsion wire w is made fast by being 

 wound three times round the little reel r and the end secured 

 by a needle-point. The torsion wire passes over a zinc wheel 

 about 13 feet above the beam B, and down again to the same 

 level, where it passes round the zinc screw z ; so that by 

 turning this screw the vessel A can always be brought to the 

 same level, whatever be the specific gravity of the liquid 

 within it. To the top of this vessel are attached two glass 

 arms, one on each side, one carrying a photographically- 

 reduced scale, s, on glass, and the other a counterpoise. The 

 vessel A is entirely enclosed in a screen shown dark in the 

 Plate. The thick portions of the screen are of wood; but the 

 thin cylindrical part immediately round A is of thin ebonite, 

 and the hemispherical part below A is of glass. This screen 

 prevents the whirlwind of air caused by the revolving magnets 

 from interfering with the motion of the vessel A. It also 

 supports a microscope, H, provided with cross-wires, which 

 is used to read the position of the scale S. The magnet con- 

 sists of 24 semicircular bars put together as shown, so as to 

 form a ring, the similar poles of the different bars being kept 

 from touching one another by the fourteen pieces of iron 

 shown dark in the plan. The magnet is 1\ in. in external 

 diameter, and weighs about 18 lbs. The magnetic field within 

 it is remarkably uniform ; iron-filings on glass take the form 

 shown by the streak lines in the plan. The magnets are at- 

 tached by four brass plates to the gun-metal wheel, G, which 

 screws against an accurately turned shoulder on the vertical 

 steel shaft, I. This shaft runs in cylindrical split gun-metal 

 bearings; but its weight is taken by a slightly convex hardened 

 steel round-headed pin let into the shaft and resting on a plate 

 of quartz. The framework which carries the bearings consists 

 of five triangles of cast iron screwed together in the form of 

 a distorted octahedron, a form which gives great rigidity ; it 

 is omitted in the Plate to avoid complication ; but its general 

 outline is shown by dotted lines .s. The lower triangle of 

 the framework is screwed to a thick slab of mahogany, which 

 is clamped to the stone mantlepicce of the room; and one 



