Edison Effect in Glow Lamps. 63 



positive electrode of the lamp is represented by the magnitude 

 of the vertical ordinate of each curve. For every one of the 

 different voltages at which the lamp is worked, there is there- 

 fore a curve representing by its ordinates this current strength 

 through a galvanometer inserted between the middle plate, 

 placed at these positions, and the positive electrode of the 

 lamp, and it is seen that there is a minimum value for this 

 current at a position equal to } of the whole distance between 

 the legs reckoned from the negative leg. 



Imagine the middle plate therefore connected through a 

 galvanometer with the positive electrode of the lamp, and let 

 the middle plate be first placed close to the positive leg and 

 then moved continuously nearer towards the negative leg. 

 The current through the galvanometer would first fall off as 

 the plate receded from the positive leg, and after reaching 

 a minimum at a point about \ of the whole distance between 

 the legs reckoned from the positive leg, would rise up to a 

 maximum when the middle plate was as nearly in contact with 

 the negative leg as possible without actually touching it. 



§ 6. Experiment 3. — In order to explore more thoroughly 

 the action of the different portions of the incandescence carbon 

 conductor in producing this effect, a lamp was taken having 

 a horse-shoe shaped carbon, and a pair of small platinum cylin- 



Fig. 5. 



ders, held on platinum wires sealed through the glass, so 

 placed as to embrace without touching the carbon conductor. 

 One of these cylinders, X, was placed so as to embrace the 

 carbon near the bottom of the leg, and the other, Y, near the 

 spring of the arch (see fig. 5). These small cylinders had a 



