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



47 



Fig. 15. 



nection with the positive electrode of the lamp carbon, the negatively- 

 charged molecules will discharge themselves against it. A plate so 

 placed may catch more or less of this stream of charged molecules 

 which pour across between the heels of the carbon loop. There are 

 many extraordinary facts, which as yet I have been able only imper- 

 fectly to explore, which relate to the sudden changes in the direction 

 of the principal stream of these charged molecules, and to their 

 guidance under the influence of magnetic forces. The above rough 

 sketch of a theory must be taken for no more than it is worth, viz. 

 as a working hypothesis to Suggest further experiments. 



These experiments with incandescence lamps have prepared the 

 way for me to exhibit to you some curious facts with respect to the 

 electric arc, and which are analogous to those which we have passed 

 in review. If a good electric arc is formed in the usual way, and if 

 a third insulated carbon held at right angles to the other two is placed 

 so that its tip just dips into the arc {see Fig. 15), we can show a similar 

 series of experiments. It is rather 

 more under control if we cause 

 the arc to be projected against 

 the third carbon by means of a 

 magnet. I have now formed on 

 the screen an image of the carbon 

 poles and the arc between them, 

 in the usual way. Placing a 

 magnet at the back of the arc, I 

 cause the flame of the arc to be 

 deflected laterally and to blow 

 against a third insulated carbon 

 held in it. There are three in- 

 sulated wires attached respec- 

 tively to the positive and to the 

 negative carbons of the arc, and 

 to the third or insulated carbon, 

 the end of which dips into the 

 flame of the arc projected by the 

 magnet. On starting the arc 

 this third carbon is instantly 

 brought down to the same elec- 

 trical potential as the negative carbon of the arc, and if I connect 

 this galvanometer in between the negative carbon and the third or 

 insulated carbon I get, as you see, no indication of a current. Let 

 me, however, change the connections and insert the circuit of my 

 galvanometer in between the positive carbon of the arc and the middle 

 carbon, and we find evidence, by the violent impulse given to the 

 galvanometer, that there is a strong current flowing through it. The 

 direction of this current is equivalent to a flow of negative electricity 

 from the middle carbon through the galvanometer to the positive 

 carbon of the arc. We have here then the " Edison efi*ect " repeated 



Electric arc projected by magnet against 

 a third carbon, and showing a strong 

 electric current flowing through a 

 galvanometer, G, connected between 

 the positive and this third carbon. 



