1890.] on Problems in the Physics of an Electric Lamp, 



37 



average without meeting with interference by collision with another 

 molecule, and the facts revealed to us by these shadows show that this 

 must be the case. I have also at hand some Edison lamps in which 

 these " molecular shadows " are finely shown, but in these cases the 

 deposit on the interior of the bulb is not carbon but copper, because 

 the molecular scattering has here taken place by excessive temperature 

 developed at the copper clamps by which the carbon filament is 

 attached to the platinum wires. The theory, however, is the same. 

 The deposit of copper shows a fine green colour by transmitted light 

 in the thinner portions. One curious lamp also before me had by an 

 accident an aluminium plate volatilised within the bulb. The glass 

 receiver has in consequence been covered with a mirror-like deposit 

 of aluminium, which on the thinner portions shows a fine blue colour 

 by transmitted light, and a silvery lustre by reflected light. This 

 lamp also shows a fine " molecular shadow." 



These facts prepare us to accept the view that when a glow-lamp 

 is in operation the highly rarefied residual air in the interior of the 

 bulb is being traversed in all directions by multitudinous carbon 

 atoms projected off from the incandescent carbon conductor. I now 

 wish to pass in review before you some facts which indicate that these 

 carbon atoms carry with them electric charges, and that they are 

 charged, if at all, with negative electricity. I may preface all by 

 saying that much of what I have to show 

 you will be seen to be closely related to 

 the phenomena studied by Mr. Crookes in 

 his splendid and classical researches on 

 radiant matter. Our starting-point for 

 this purpose is a discovery made by 

 Mr. Edison in 1884, and which received 

 careful examination at the hands of Mr. 

 Preece in the following year,* and by 

 myself more recently. Here is the initial 

 experiment. A glow-lamp having the usual 

 horseshoe-shaped carbon (see Fig. 3) has a 

 metal plate held on a platinum wire sealed 

 through the glass bulb. This plate is so 

 fixed that it stands up between the two 

 sides of the carbon arch without touching 

 either of them. We shall illuminate the 

 lamp by a continuous current of elec- 

 tricity, and for brevity's sake speak of 

 that half of the loop of carbon on the 

 side by which the current enters it as the positive leg, and the other 

 half of the loop as the negative leg. The diagram in Fig. 4 shows 



Fig. 3. 



Glow-lamp having insulated 

 metal middle plate M 

 sealed into bulb to exhibit 

 " Edison effect." 



* Mr. Preece's interesting paper on this subject is published in the 

 ' Proceedings' of the Royal Society for 1885, p. 219. See also ' The Electrician,' 

 April 4th, 1885, p. 436. 



