88 Prof. J. A. Fleming on the 



o 



also have shown that the magnitude of the current flowing 

 through the galvanometer is increased by bringing the plate 

 near to the base of the negative leg, or, better still, making 

 the metal plate in the form of a cylinder and making this 

 cylinder surround the negative leg near its base ; and cor- 

 respondingly it is diminished by removing it from the 

 negative leg to a considerable distance, or by shielding this 

 collecting-plate from the radiation from the negative leg of 

 the carbon. The experiments with the condenser have also 

 given evidence that when an insulated metal plate is sealed 

 into a lamp this plate is brought down either instantly 

 or in a very short period of time to the potential of the 

 negative leg near its base or to that of the negative electrode 

 of the lamp. In looking for an explanation of these facts we 

 are assisted by our previous knowledge that in carbon incan- 

 descence lamps, when working at an efficiency equivalent to 

 3 to 4 watts per candle-power, there is a gradual loss of 

 carbon from all parts of the conductor. We know also that 

 the carbon molecules which are projected from the conductor 

 are thrown off into a space so highly vacuous that their mean 

 free path is of a length comparable with, or greater than that 

 of the dimensions of the glass bulb. The existence of 

 molecular shadows in incandescence lamps* affords evidence 

 that from intensely heated portions of the carbon conductor 

 carbon molecules are projected in straight lines and move 

 freely forward until they impinge against the glass. Com- 

 mercial experience informs us that at and above a tem- 

 perature corresponding to 3 watts per candle-power this loss 

 of carbon becomes very rapid and thins away the filament 

 , in one place, or generally reduces the diameter of the carbon 

 , conductor. Hence we have every reason to believe that 

 when in a normal state of incandescence the carbon conductor 

 i in a lamp is throwing off in all directions carbon molecules, 

 and that in the vacuum usually obtained the mean free path of 

 these projected molecules is comparable with the dimensions 

 of the vessel containing the conductor. The whole of the 

 experiments which are detailed here seem to be capable of 

 consistent interpretation if we may justifiably make the 

 hypothesis that these carbon molecules or atoms so projected 

 from the conductor when intensely heated by the current 

 flowing through it are all negatively charged. Some of the 

 observed facts seem to point to the conclusion that the 

 molecules projected from the incandescent conductor, whether 

 they are portions of the conductor itself or molecules of the 

 residual gases, respectively carry away negative charges 

 * See J. A. Fleming, ' Philosophical Magazine/ August 1885, p. 141. 



