Edison Effect in Gloiv Lamps. 91 



electrodes one of which is incandescent and the other of 

 which is cold. Negative electricity is discharged at once 

 out of the hot surface but not out of the cold, and a 

 negative discharge can take place from hot to cold but not 

 vice versa. When the discharge of a charged condenser is 

 effected by connecting the positive plate, through a galvano- 

 meter, with a metal plate sealed into the lamp and the 

 negative plate with the lamp circuit, and then switching on the 

 lamp, there is a curious instant of delay before the discharge 

 begins. When the metal plate is placed very near the negative 

 leg of the carbon the discharge of the condenser is complete 

 in one instant. This the case when a lamp of the type No. 4 

 (fig. 18) is used. If, however, we employ a lamp of the type 

 No. 2 (fig. 10), in which the metal plate is at some distance 

 from the negative leg of the carbon, the discharge of the 

 condenser is long drawn out and the electric charge in it is as 

 it were tapped off slowly and not in one short sharp discharge. 



Moreover this effect of discharging a condenser takes place 

 only when the carbon is above a fair red heat. At brilliant 

 incandescence and when the carbon is above a temperature 

 corresponding to 3 watts per candle-power, the discharging 

 power of a lamp of the type of No. 4 is very great. A con- 

 denser of 10 or 20 microfarads capacity charged to 50 volts 

 is discharged instantly if its positive plate is connected to the 

 metal plate placed not far from the negative end of the 

 incandescent carbon conductor. 



The foregoing results were confirmed with lamps of other 

 types. Using, for instance, a lamp like No. (i with the 

 aluminium plate placed outside the carbon horse-shoe and. 

 near the leg, the same discharging power for positive elec- 

 tricity was found. It was not dependent on the direction of 

 the current through the lamp carbon, although it seemed a 

 little more vigorous when the leg nearest the plate was the 

 negative leg. As above observed, the rate of discharge was 

 much reduced when employing a lamp having the metal plate 

 placed edgeways on to the carbon and some way from it, as, 

 for instance, when employing a lamp of the form of No. 2. 



§ 22. Experiment 19. — A series of experiments was in this 

 case also tried to determine the effect of shielding the negative 

 leg of the carbon. The lamp no. 9 was employed, in which 

 one leg of the carbon was enclosed in a glass tube connecting 

 the positive plate of a charged condenser through a galvano- 

 meter with the middle plate of the lamp, and the negative 

 plate of the condenser somewhere to the battery circuit ; it 

 was found that when the shielded leg of the carbon was the 

 positive leg the condenser was discharged as before. If, 



