Magneto-Electric Induction. 459 



A 5-in. sulphuric-acid dish was held just above it by a separate 

 stand to avoid any effects that vibration might cause ; and into 

 this dish the liquid to be tested could be put ; a cover was also 

 put over the dish. Mercury was first tried ; and it soon began 

 to rotate in the same direction as the magnet. Dilute sul- 

 phuric acid of the best-conductivity strength was next expe- 

 rimented on : a wood float that had been soaked in paraffin to 

 make it float in the middle of the vessel served for an index 

 to show motion of liquid. The float followed the magnet at 

 the rate of about 15° an hour, but was always rather uncertain 

 in its movement. The only noticeable feature of this experi- 

 ment was, the fine sulphate-of-lead powder which had settled 

 during the night arranged itself during the experiment in 

 fine concentric rings, the circle immediately above the poles of 

 the electro-magnet being bare. A solution of sulphate of 

 copper showed no indications whatever. 



As this method did not seem at all promising, the apparatus 

 shown in vertical section in fig. 2 was next used. A glass 

 bulb B was filled with the same sulphuric acid that was used 

 in the last experiment, and hung by a silk thread in a small 

 beaker c which acted as a screen ; there was also a screen 

 above. The electro -magnet e was made to revolve before the 

 current was turned on, to see if there was any mechanical 

 effect; but there was none at all ; as soon, however, as the wires 

 were connected with the battery the bulb B began to swing 

 round, and stopped at about 180°. Here there was an unmis- 

 takable effect obtained with a non-metallic liquid conductor. 



If, then, the torsion is proportional to the conductivity of the 

 liquid in the vessel, which it undoubtedly is, we have the means 

 of measuring the conductivity of liquids without using elec- 

 trodes, and therefore without polarization. To see how the 

 method worked, the preliminary apparatus shown in figs. 3 and 

 4 was constructed. N S is a double electro-magnet made from 

 iron bar 1 in. wide and J thick, bent as shown in horizontal 

 section in fig. 3, so that the two poles are inside : with this 

 arrangement the lines of force which intersect the cylindrical 

 vessel B are almost straight and parallel. The vessel B was 

 hung in the cylindrical screen c by a platinum wire 15 ft. long 

 and 0*002 in. in diameter; and a small mirror was fixed to it for 

 observing the reflexion of a small gas-flame. The liquid could 

 be introduced by a pipette through a small hole in the side. 

 Mixtures of sulphuric acid and water were made in the fol- 

 lowing proportions by weight, and were tested in the above 

 apparatus : — 



a. b. c. d. e. 



Acid 7 6 5 4 3 



Water 3 4 5 6 7 



212 



