64 



Prof. J. A. Fleming on the 



length of about 12 millims. and a diameter of about 8 millims. 

 so that the distance from the carbon filament to the inner 

 surface of the cylinder was about 3 or 4 millims. The lamp 

 had a rather thick carbon, and at an electromotive force of 

 48 volts took a current of 1'32 amperes to raise it to its 

 normal incandescence of 18*8 candles, corresponding to 3*3 

 watts per candle-power. This lamp will be alluded to as 

 Lamp No. 3. 



It is obvious that there are four possible arrangements in 

 which a current can be obtained between an embracing 

 cylinder and a positive electrode of the lamp. These are 

 illustrated in fig. 6, in which the horse-shoe shaped line stands 



Fig. 6. 



Position Position Position 



(1). (2). (3). 



LAMP NO 3 



Position 

 (4). 



for the carbon filament, X and Y are the platinum cylinders, 

 P and N are the positive and negative electrodes of the lamp, 

 and Gr is the galvanometer. 



We will call these arrangements (1), (2), (3). (4), as figured. 

 It will be seen that if we imagine the carbon filament straight- 

 ened out, these four arrangements are equivalent to being 

 able to slide a cylinder along the filament into four positions, 

 and in each position measuring the potential-difference be- 

 tween the cylinder and the positive end of the carbon. We 

 are thus able to place an embracing collecting-plate at four 

 different places along the carbon conductor, and determine 

 the potential-difference between this embracing cylinder and 

 the positive electrode of the lamp. A series of experiments 

 was made with lamp No. 3, in which the working volts of the 

 lamp were raised to various values, and in each case the 

 potential-difference between one of the cylinders X or Y and 



