On the Edison Effect in Glow Lamps. 53 



by Mr. W. H. Preece in 1885, and he subjected it to a 

 systematic examination by the aid of a number of lamps 

 having such metal plates placed in various positions *. By 

 this observer a number of very interesting facts were col- 

 lected, the result of which was to point out the general nature 

 of the phenomenon. A sufficient number of new questions 

 were, however, suggested by the information so acquired to 

 invite further inquiry. Whilst confirming and re-examining 

 the experimental results obtained by Mr. Preece, some facts 

 that had previously escaped notice presented themselves, 

 which it is the object of this paper to describe. 



§ 2. The first experiments were made with a lamp of the 

 form shown in fig. 2, similar to some used by Mr. Preece in 

 his experiments. A metal plate, generally of aluminium, is 

 supported on a platinum wire sealed through the bulb or 

 glass receiver, the plate being so fixed that its plane is at 

 right angles to the plane of the loop of the carbon, and as 

 nearly as possible midway between the legs. The plate there- 

 fore projects between the legs of the horse-shoe carbon, and 

 the carbon conductor arches over it without touching it. 

 The plate is entirely insulated from the carbon. 



The preliminary experiments with this normal type of 

 middle-plate lamp consisted in determining the effective 

 potential-difference between the third terminal and one or 

 other of the two electrodes of the carbon filament when the 

 lamp was subjected to varying steady electromotive forces 

 sufficient to raise the temperature of the carbon from dull red 

 to vivid incandescence, and in determining the magnitude of 

 the current flowing in a circuit connecting the middle plate 

 with one or other of the electrodes of the lamp. 



§ 3. Experiment 1. — An ordinary carbon filament electric 

 lamp, having the horse-shoe shaped conductor, had a platinum 

 plate (see fig. 3) about 2*5 centimetres long by 1*5 centi- 

 metres wide welded to a platinum wire, sealed through the 

 side of the bulb. The plate was so placed as to project 

 between the legs of the carbon conductor, having its plane at 

 right angles to the plane of the horse-shoe, and initially fixed 

 about halfway between the two legs. This lamp will be de- 

 scribed in the subsequent paragraphs as Lamp No. 4. Under 

 a steady electromotive force of 40 volts, this lamp took 3*7 

 amperes when working at the normal temperature corre- 



* "On a Peculiar Behaviour of Glow-Lamps when raised to High 

 Incandescence," by W. H. Preece, F.R.S. Proceedings of the Royal 

 Society, 1885, p. 219. 



