Intelligence and Miscellaneous Articles. 151 



We shall here cite only two experiments, which give the number 

 of turns of the swivel in 30 seconds in atmospheric air, and in hy- 

 drogen, at various degrees of pressure. 



In Air. In Hydrogen. 



Pressure. Number of 



millims. turns, 



10 11 



8 16 



5 25 



4 30 



Pressure. Number of 



millims. turns. 



3S.. 32 



18 46 



1 54 



In the last experiment the jet was no longer visible ; there was 

 only a slightly luminous circular sheet between the central knob and 

 the metallic ring, which, turning rapidly under the magnetic action, 

 carried the swivel with it. 



A numerous series of experiments was then devoted to the study 

 of the variations in intensity to which the current producing the jet 

 is subject when the latter turns the swivel. To measure these va- 

 riations, we employed the same derivation-apparatus which we used 

 in our preceding work, and which M. de la Rive has described in 

 his first memoir on the subject. 



We thus ascertained that the intensity of the current sensibly di- 

 minishes when the jet sets the swivel going. In air at 9 millims. 

 pressure the motion given to the swivel by the jet induced a dimi- 

 nution of 10° in the deflection of the galvanometer, viz. from 42° to 

 32°, and in air at 8 millims. a diminution of 11°, viz. from 45° to 

 34°. In another experiment, with the air at 14 millims. pressure, 

 the swivel making 18 turns in 30 seconds, and the jet 72 turns, 

 the deflection fell 8°, or from 32° to 24°. 



With hydrogen the decrease of intensity is less sensible than with 

 air ; this results from various causes — in particular, from the greater 

 electric conductivity of that gas. In the most favourable conditions 

 it is scarcely more than 5° ; and with high degrees of rarefaction it 

 is still less. 



From the diminution of intensity produced by the resistance of 

 the swivel, we presumed that, without the latter, the resistance which 

 the surrounding gaseous medium must oppose to the jet in its rota- 

 tion would by itself produce the same effect, although to a less de- 

 gree. This, in fact, was confirmed by experiment. Thus in the 

 large bell containing air at 19 millims. pressure, we obtained a di- 

 minution of 4° in the deflection of the galvanometer, it having fallen 

 from 43° to 39°. The velocity of rotation of the jet was 45 turns 

 in 30 seconds. On reducing the pressure to 8 millims., the velocity 

 of the jet being 87 turns in the same time, we had equally a dimi- 

 nution of intensity of 4°. Here the increase in the velocity of the 

 jet (which was about double) compensated the diminution in the 

 mass of the gas (which was about three fifths less). 



In determining the diminution of intensity of the current with and 

 without the employment of the swivel, we obtained with atmosphe- 

 ric air a diminution of 8° (from 28° to 20°) at 18 millims. pressure. 



