162 



Professor J. A. Fleming 



[May 21. 



In 1883 Mr. Edison for some purpose placed in the glass bulb of 

 one of his carbon filament lamps a metal plate which was carried on 

 a platinum wire sealed through the glass. When the filament was 

 rendered incandescent by a current from a battery, he found that if 

 the plate was connected by a wire, external to the lamp, with the 

 positive terminal of the filament, a small electric current flowed 

 through it. but if connected to the negative terminal no current, 

 or at most a very feeble current, flowed (Fig. 1). 



This new and interesting effect became known as the " Edison 

 effect " in glow lamps, but Mr. Edison gave no explanation of it and 

 made no practical application of it in telegraphy, or for any other 

 important purpose. 



Edison supplied some lamps with plates in the bulb to the late 



Fig. 1. — Scheme of Circuits foe Exhibiting the 

 Edisox Effect. 



Sir William Preece, and the latter found that the current called the 

 Edison effect current increased very rapidly as the filament was 

 heated to higher and higher temperatures, and that the collecting 

 plate could be placed a long way from the filament, even at the end 

 of a side tube, without altogether causing it to vanish. 



At a little later date I took up the subject, convinced that there 

 was yet much to learn about it, and one of the first things discovered 

 was that the Edison effect was greatly reduced if that side of the 

 carbon loop filament in connection with the negative pole of the 

 battery was enclosed in a glass or metal tube, or if a sheet of mica 

 was interposed between the filament and the collecting plate. This 

 seemed to indicate that the effect was due to some material emission 

 from the hot filament. 



