Edison Efect in Glow Lamps, 91 



pole to the galvanometer terminal, the current is barely if at 

 all increased. In this case the negative pole of the Clark cell 

 is in connexion with a cold metal electrode and the positive 

 pole is in connexion through the galvanometer with the 

 incandescent carbon electrode, and under these circumstances 

 the galvanometer detects no current flowing. The position 

 of the Clark cell is now reversed, and it is joined up so that 

 its positive pole is in connexion with the middle plate and its 

 negative pole in connexion, through the galvanometer, with 

 the incandescent carbon electrode. It is then found that a 

 considerable current of some few milliamperes in magnitude 

 is flowing through the galvanometer. The direction of this 

 current in the ordinary way of speaking is from the negative 

 electrode of the lamp through the galvanometer to the metal 

 plate sealed into the bulb. We thus find that a negative 

 current of electricity can be made to flow across the vacuous 

 space between the incandescent carbon and the metal plate 

 from the hot carbon to the cooler metal plate, but not in the 

 reverse direction. The space presents an apparently uni- 

 lateral conductivity. 



§ 27. Experiment 25. — The same experiment was repeated, 

 only using instead of a Clark cell an insulated secondary 

 battery of 25 small cells. When the secondary battery 

 (see fig. 21) was connected with its negative pole to the 

 metal plate and its positive pole through the galvanometer to 

 the negative electrode of the carbon, no current greater than 

 that found with the Clark cell similarly arranged was found ; 

 but if the secondary battery was reversed and joined up with 

 its positive pole to the middle plate and its negative pole 

 through the galvanometer to the negative electrode of the 

 incandescent carbon, then so strong a current flowed through 

 the galvanometer that it could not be measured without 

 shunting-down the galvanometer considerably. The same 

 experiments were repeated with the lamps having the zigzag 

 wire as a metal plate, No. 7, and the same general results 

 obtained. These experiments therefore show that in a circuit 

 which consists partly of a galvanometer- wire and partly of a 

 highly vacuous space bounded by two electrodes— one a metal 

 plate and the other an incandescent carbon surface,— the 

 insertion of an electromotive force in one direction can produce 

 a very sensible current, but that if the electromotive force is 

 reversed then no current flows. The direction of the electro- 

 motive force must be such as to urge negative electricity from 

 the hot surface to the cold across the vacuum space. 



§ 28. Experiment 26. — In order to make use of different 

 parts of the incandescent conductor as the electrode opposed 



Phil Mag, S. 5. Vol. 42. No. 254. July 1896. H 



