Edison Effect in Glow Lamps. 89 



proportional in magnitude to the potential of the conductor 

 at the point from which they are thrown off. They may, 

 therefore, be looked upon as condensers of small but definite 

 electrostatic capacity charged to the potential (negative) 

 of that part of the incandescent conductor at which they 

 separate from it. We have then in addition to explain 

 how it comes to pass that there are few or no projected 

 molecules charged positively. Two suggestions may be made 

 on this point : either the radiation of matter is wholly confined 

 to that half of the conductor at a negative potential or the 

 incandescent carbon molecule thrown off from the heated 

 conductor cannot retain a positive charge. There is much 

 to lead to the conclusion that from all parts of the incan- 

 descent carbon conductor there is a constant radiation of 

 matter carrying a negative electric charge. The nearer down 

 to the negative electrode of the carbon we select our point 

 of observation the greater is this molecular charge found to 

 be. It will be convenient to denote this conveyance of 

 electric charge by moving charged molecules by the term 

 molecular electrovection. We can then state the hypothesis 

 thus — from all portions of the negative leg of the carbon 

 loop a process of molecular electrovection is going on when 

 the conductor is incandescent, the molecular charge being- 

 negative, and equal in potential to that of the point on the 

 conductor from which it is projected. 



§ 21. On the assumption that a molecular shower of nega- 

 tively charged atoms was being projected against the middle 

 plate when the conductor of the lamp was incandescent it 

 was considered probable that a positively charged conductor 

 connected to the middle plate would be discharged, and this 

 was found to be the case. 



Experiment 18. — A lamp of the form of No. 4, having a 

 middle metal plate placed between the carbon legs, had its 

 middle plate connected to one terminal of the Elliott gal- 

 vanometer. The other terminal of the galvanometer was 

 connected to one terminal of a condenser of 5 microfarads 

 capacity. The other terminal of the condenser was connected 

 by a wire to the gas pipes of the laboratory. The lamp was 

 actuated by secondary batteries (see fig. 17) not very well 

 insulated. If the condenser was charged to a potential of 

 50 volts so that the plate next the galvanometer was posi- 

 tively charged, then this positive charge was instantly dis- 

 charged when the carbon was rendered incandescent. If, 

 however, the plate of the condenser in connexion with the 

 middle plate through the galvanometer was charged negatively 

 the condenser was not discharged when the lamp was illumi- 



