Edison Effect in Glow Lamps. 73 



Experiment 8. — A normal 100-volt carbon-filament lamp, 

 having a carbon filament coiled in a spiral of two turns (see 

 fig. 7) bad a short stout platinum wire (*024 inch diain.) 



Fig. 7. 



sealed across the bulb so as to thread through, without 

 touching, the spirals of the carbon. The lamp at 100 volts 

 took 1*54 amperes and gave an illumination of 40 candles, 

 equivalent to a power absorption of 3*9 watts per candle-power. 

 The vacuum was very good. This lamp will hereafter be 

 called Lamp No. 1. As before, no current could be detected 

 by a galvanometer when joined up between the platinum wire 

 and the negative electrode, but when the galvanometer was 

 connected between the platinum wire and the positive elec- 

 trode of the lamp a current of some milliamperes was found 

 passing through it. As in the case of lamp No. 4, this lamp 

 was characterized by a great tendency to change suddenly 

 the value of the current flowing through the galvanometer 

 when the working volts on the lamp were kept perfectly con- 

 stant. In the first series of observations the milamperemeter 

 was employed to measure the current flowing between the 

 positive electrode of the lamp and the platinum wire when it 

 was connected between these points, and at and beyond a 

 working-pressure of 90 volts or so the galvanometer would 

 often jump suddenly from one reading to another, when the 

 lamp working volts were kept perfectly constant. 



In the following table, No. 9, are collected the results when 

 the working pressure of the lamp was graduallv raised from 

 80 to 100 volts :— 



