Edison Effect in Glow Lamps. 87 



surface of the metal middle plate, and when placed up against 

 it it shielded one surface from, and when jerked on one side 

 it exposed that surface to, the carbon leg opposite to that 

 surface. This lamp (called No. 5) was set on the circuit in 

 the first place so that the leg of the carbon horse-shoe 

 opposite to the mica-shielded side of the middle plate was 

 the positive leg. If the carbon was brought up to an incan- 

 descence corresponding to about 3*5 or 4 watts per candle- 

 power and the galvanometer connected between the positive 

 electrode and the middle plate, then it w r as found that the 

 effect on the galvanometer current which was produced by 

 the interposition or withdrawal of the screen of mica between 

 the positive leg and the plate was not very great. It reduced 

 the current through the galvanometer from about '44 mil- 



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liampere to *38 milliampere. If, however, the current 

 flowing through the lamp carbon is reversed in direction so 

 that the mica screen is interposed on that side of the middle 

 metal plate which faces the negative leg, the result is very 

 different. When the screen is down, the current flowing 

 through the galvanometer from the positive electrode to the 

 middle plate being as before "44 milliampere, the interposition 

 of the mica screen on the side of the plate facing the 



negative leg reduced the current at once to zero. We find 



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therefore, that the interposition of a mica screen between the 

 middle plate and the negative leg reduces to zero the current 

 flowing between the positive lamp electrode and the middle 

 plate. As in all other lamps with a middle plate set exactly 

 between the legs, the current through the galvanometer 

 joining the middle plate and positive electrode is very liable 

 to "jump " from a low to a higher value or vice versa. When 

 the current has its higher value corresponding to any given 

 voltage on the lamp terminals, the effect of screening is less 

 marked, and although the interposition of the mica screen on 

 the side facing the negative leg has an effect of reducing 

 the current flowing through the galvanometer connected 

 between the middle plate and positive electrode it is not by 

 any means reduced to zero. 



§ 20. The foregoing experiments afford proof that the 

 production of the current through a galvanometer joined 

 between the positive electrode of the lamp and a metal plate 

 placed somewhere in the vacuous bulb, is an effect due chiefly 

 to the negative leg of the carbon, and that shielding the 

 negative leg by enclosing it in a glass or metal tube, or 

 covering with a mica screen that surface of the plate which 

 is exposed to the negative leg, either quite prevents or greatly 

 reduces the production of this current. The experiments 



