60 Prof. J. A. Fleming on the 



higher one, and this occurs when the working pressure of the 

 lamp is preserved steady. There is also an effect produced 

 by the presence of a magnet near the lamp bulb. When the 

 current is at the low value corresponding to any working 

 voltage, the galvanometer reading does not seem to be per- 

 ceptibly altered by bringing a magnet near the lamp, but 

 when it is at its high value, the reading is sometimes increased 

 for a little, showing a steady deflexion, and then immediately 

 falls to its low value. 



§ 5. Experiment 2. — The difference of potential between 

 the middle plate and the positive electrode of the lamp depends 

 to a considerable extent upon the position of the middle plate. 

 Supposing the plate to be placed with its plane perpendicular 

 to the plane of the carbon horse-shoe and then moved to 

 various positions between the two legs of the carbon, it is 

 found that the difference of potential between the plate and 

 the positive electrode will have different values according to 

 the position of the plate. This fact was elucidated by means 

 of the same lamp No. 4 as used above. By carefully tapping 

 the lamp, the supporting platinum wire carrying the platinum 

 middle plate could be bent so as to displace the plate from its 

 symmetrical position as regards the two carbon legs, and 

 bring it nearer to one or other of the legs. In several 

 different positions the current flowing through the milampere- 

 meter, when connected between the middle plate and positive 

 electrode, was measured, the lamp being kept meanwhile at 

 the same working electromotive force. 



Estimating as nearly as possible the fractional distances, 

 the plate was placed at distances from the negative leg equal 

 to ^q, J, -J, f , and ^q of the whole distance between the 

 positive and negative legs, and the lamp being taken through 

 a definite cycle of volts, the potential- difference between the 

 middle plate and the positive electrode was measured with 

 the milamperemeter. The results are collected in the follow- 

 ing tables. The diagrams in fig. 6 represent the horse-shoe 

 carbon loop and the middle plate M in various positions, the 

 galvanometer G being inserted between the plate M and the 

 positive electrode P. By the phrase " whole distance " in 

 the following tables is meant the whole distance or width 

 of the space between the positive and negative carbon 

 leg :— 



