Edison Effect in Glow Lamps. 55 



milamperemeter by a more sensitive Elliott mirror galvano- 

 meter (resistance 7142 ohms), it was found that a small 

 current passed through it, when joined in between the negative 

 electrode of the lamp and the middle plate, but that this 

 current had a magnitude hardly exceeding '0001 of a milli- 

 ampere when the lamp was at its normal incandescence. 



In order to avoid repetition, it may be here said that, unless 

 otherwise stated, the terminal of the lamp in connexion with 

 the positive pole of the working battery will be spoken of as 

 the positive electrode of the lamp; that in connexion with the 

 negative pole of the battery as the negative electrode. For 

 brevity's sake, the half of the carbon filament between the 

 centre of the filament and the positive electrode will be called 

 the positive leg, and the other half the negative leg. 



§ 4. A preliminary series of experiments was made with lamp 

 No. 4 by placing the lamp in a photometer and determining 

 the watts per candle-power and the current taken by the 

 lamp corresponding to various working electromotive forces, 

 taken over the whole range of electromotive force from that 

 necessary just to render the filament incandescent to the 

 highest the lamp could with safety endure. In any subse- 

 quent experiments, the simple measurement of the potential- 

 difference between the electrodes of the lamp enabled the 

 rate of dissipation of energy in the filament and the watts per 

 candle-power to be deduced. It may here be remarked that 

 in the preliminary experiments some difficulties arose from 

 the occlusion of residual gas by the middle metal plate, but 

 finally this was overcome, and the vacuum in these experi- 

 mental bulbs made and preserved as perfect as in good ordinary 

 commercial lamps. The following results were then obtained 

 with this lamp No. 4. The lamp was raised to various degrees 

 of incandescence by varying the working volts by the aid of 

 a rheostat in series with the lamp. 



The milamperemeter was employed to measure the effective 

 potential- difference between the positive electrode of the lamp 

 and the middle plate and then, tabulating against the working- 

 volts of the lamp the current in milliamperes flowing through 

 the galvanometer, the potential-difference between the middle 

 plate and the positive electrode of the lamp was calculated 

 from these figures. The results are given in the table below 

 (Table No. 1). 



