100 Prof. J. A. Fleming on the 



§30. Experiment 28. — The effect of lowering the vacuum 

 was also the subject of experiment. In a lamp with a highly 

 perfect vacuum no current greater than about *0001 milli- 

 ampere is observed when a very sensitive high-resistance 

 galvanometer is joined up between the negative electrode of 

 the carbon and the insulated middle plate, and, as we have 

 seen, the experiments with the electrostatic voltmeter showed 

 that the plate was brought down under these circumstances to 

 the potential of the base of the negative leg of the carbon. If, 

 instead of employing a very perfect vacuum, a bad one is pro- 

 duced by imperfectly exhausting the lamp, then it is found 

 that under these conditions the Elliott galvanometer indicates 

 a not inconsiderable current of something approaching to a 

 milliampere when joined in between the negative electrode of 

 the lamp and the middle plate. Hence, when the vacuum is 

 imperfect the equality in potential between the middle plate 

 and the negative electrode is not maintained. 



The direction of the current in this last case is such as to 

 show that negative electricity is flowing through the galvano- 

 meter from the negative electrode of the lamp to the middle 

 plate. In other words, negative charge is carried over from 

 the plate to the positive leg of the carbon across the imperfectly 

 vacuous space ; and the means by which this is effected is the 

 residual air. This seems to afford proof that the normal effect 

 of the molecular electrovection of negative electricity from the 

 negative leg is due to carbon molecules, and that the presence 

 of residual air exhibits itself, when present beyond a certain 

 amount, in producing an effect wdiich the carbon molecular 

 electrovection cannot produce. 



§ 31. Experiment 29. — It seemed very desirable to ascertain 

 if the effect of molecular electrovection exists in the case of 

 an incandescent platinum wire rendered vividly incandescent 

 in a highly perfect vacuum. A bulb w T as accordingly con- 

 structed similar in every way to lamp No. 4, but having a 

 platinum-wire horse-shoe conductor and a platinum middle 

 plate. When this wire was rendered highly incandescent by 

 a continuous current, a sensitive galvanometer (the high- 

 resistance Elliott) showed a current of about one five- 

 thousandth of a milliampere when connected between the 

 positive electrode of the incandescent wire and the middle 

 plate, but little or no current when connected between the 

 negative electrode and the middle plate. This molecular 

 electrovection current was thus very much less in magnitude 

 than that observed in the case of the carbon filament lamps, 

 but it is in the same direction. We are, however, enabled to 

 state that at a condition of vivid incandescence just short of 



