Edison Effect in Glow Lamps. 67 



surrounds the top of the positive leg, and is as far as possible 

 away from the foot of the negative leg. We have here a 

 confirmation of the fact observed with respect to lamp No. 4, 

 viz. : that the potential-difference between the positive lamp 

 electrode and the metal plate held somewhere near the incan- 

 descent carbon conductor is greatest when the plate is as near 

 as possible to the foot of the negative leg or negative elec- 

 trode. 



In this lamp No. 3, in which in all positions the cylinder 

 employed is placed very near some point on the incandescing 

 conductor, the current through the galvanometer joining the 

 positive electrode and the cylinder has never been observed 

 to jump or to take double values as in the case of lamp No. 4. 



If a sensitive galvanometer is connected to the two insu- 

 lated cylinders X and Y, and if in addition there is a battery 

 in series with this galvanometer, then no current can be de- 

 tected in such an arrangement even when the battery has an 

 electromotive force of 120 volts, whether the lamp carbon is 

 incandescent or not. Just as in the case of lamp No. 4, the 

 current obtained by connecting either cylinder with the nega- 

 tive electrode of the lamp is excessively small. 



In order to obtain curves showing the mode of variation of 

 the effective difference of potential between one or other of 

 the metal cylinders X and Y and the positive electrode of the 

 lamp, a set of observations was made on the lamp when sub- 

 mitted to various working voltages, and at the same time the 

 milamperemeter was connected first between the positive 

 electrode and cylinder X and then between the positive 

 electrode and cylinder Y, with the following tabulated 

 results (p. 68). 



These results, when plotted out in curves in which hori- 

 zontal ordinates represent the working volts of the lamp and 

 the vertical ones the milliampere currents going through the 

 galvanometer, in both cases have the same general form as 

 curve No. 1 plotted for the case of lamp No. 4. 



F 2 



