56 



Prof. J. A. Fleming on the 



Table No. 1. Lamp No. 4. MWamperemeter. 



Table showing the volts between the middle plate and the 

 positive electrode, and the current flowing through a 

 galvanometer of 6372 ohms resistance connecting them, 

 taken for various working voltages of the lamp. 



Working 



volts of 



lamp. 



Volts be- 

 tween plate 

 and positive 

 lamp elec- 

 trode. 



Current in 

 milli- 

 amperes 

 through gal- 

 vanometer. 



Working 



volts of 



lamp. 



Volts be- 

 tween plate 

 and positive 



electrode. 



Current in 

 milli- 

 amperes 

 through gal- 

 vanometer. 



30 



•54 



•085 



36 



10-7 



1-69 



32 



1-2 



•190 



37 



12-7 



2-01 



32 5 



1-6 



•25 



38 



14 9 



236 



33 



2-8 



•44 



39 



171 



2-71 



33-5 



4-7 



•74 



40 



18-9 



299 



34 



5-3 



•84 



41 



214 



3-37 



34-5 



7-1 



1-12 



42 



23-4 



371 



35 



7-8 



1-23 



43 



25-2 



3-99 









44 



26-8 



4-25 



The results given in table No. 1 are plotted in curve No. 1, 

 in which horizontal abscissae represent to scale the working 

 volts of the lamp and vertical ordinates the miliiampere 

 currents through the galvanometer. It will be seen that the 

 curve representing the current from lamp electrode to plate 

 takes a rather sharp turn upwards at a point corresponding to 

 33 working volts, and this occurs when the lamp is working 

 at about 7'8 watts per candle-power. Beyond this point the 

 curve is very approximately a straight line. Accordingly, at 

 and beyond the volts at which the carbon filament becomes 

 fairly well incandescent, the effective potential-difference 

 between the middle plate and the positive lamp electrode is 

 very nearly a linear function of the lamp voltage ; and at the 

 normal working volts, viz. 40 volts, this potential-difference 

 between the middle plate and the positive electrode so deter- 

 mined is apparently about half that between the lamp terminals, 

 the plate being nearly midway between the carbon legs. 

 The results given in Table 1 are the mean of several obser- 

 vations, but it was noticed that when the lamp was maintained 

 at a steady voltage, the potential-difference between the 

 middle plate and the positive electrode would often jump 



