the Conductivity of Liquid a. 331 



slightest sign of vibration reoccuring. In fact the magnets 

 spun like a great top, though the screwea-up bearing permitted 

 no shake of the shaft within it. On passing the critical 

 speed when stopping, the oscillation set up is hardly notice- 

 able. 



Before beginning the experiments it was necessary to de- 

 termine what effect, if any, the magnets had on the vessel A. 

 For this purpose it was filled with distilled water, and the 

 magnets made to run at speeds high and low ; but not the 

 slightest movement of the scale could be detected by the mi- 

 croscope so long as the magnets were revolving. Had it 

 moved the hundredth part of a millimetre, that must have 

 been seen ; but when the magnets were stopped the scale was 

 found to have moved; and as the magnets were turned round 

 to successive positions through 360°, the scale went through 

 a complete oscillation of 18 divisions. These two preliminary 

 experiments showed that, though the cell was affected by 

 placing the magnet in different positions (an effect probably 

 due to the three turns of the torsion- wire round the reel in 

 the beam having a directive action), yet electrically the cell 

 was all that could be desired, and any torsion observed during 

 the rapid revolution of the magnets must be due to the con- 

 ductivity of the liquid in the cell. iSTo currents could be in- 

 duced in the torsion-wire round the reel, for they formed an 

 open circuit ; but even if closed, the reaction of the induced 

 currents on the magnets would be inappreciable, owing to 

 the small diameter of the coil and its great distance above the 

 magnets. It is also clear that the magnets couid have no 

 mechanical action on the cell due to air-currents, vibration, 

 &c. It should be mentioned here that the screen was fixed 

 to a panel screwed to the wall, so as to be quite independent 

 of any thing connected with the magnets; also the torsion-wire 

 was hung from a bracket fixed to the wall under the cornice 

 of the room. This separation of the supports of the various 

 parts is to ensure there being no mechanical action causing 

 torsion due to vibration &c, an effect observed by Snow 

 Harris in certain of his experiments. 



Having now made clear the construction of the apparatus, 

 it will be well to explain next the principle of its action, and 

 then to describe the method of working with it, giving at the 

 same time the few results that have been obtained so far. 



When a conductor is in a moving magnetic field it is urged 

 in the direction of motion by a force which varies directly 

 as the relative movement, directly as the intensity of the field, 

 and directly as the conductivity of the conductor, i. e. if the 

 form of the conductor is always the same. In the present 



