1890.] on Problems in the Physics of an Electric Lamp. 



39 



Fig. 6. 



of the lamp, we notice that though made highly sensitive the galyano 

 meter indicates no current flowing through it whilst the lamp is in 

 action. Switching off the current from the lamp produces, as you 

 see, a violent kick or deflection of the galvanometer, indicating a 

 sudden rush of current through it. 



In endeavouring to ascertain further facts about this effect one of 

 the experiments which early suggested itself was one directed to 

 determine the relative effects of 

 different portions of the carbon con- 

 ductor. Here is a lamp (see Fig. 6) 

 in which one leg of the carbon horse- 

 shoe has been enclosed in a glass 

 tube of the size of a quill, which 

 shuts in one-half of the carbon. 

 The bulb contains, as before, an 

 insulated middle plate. If we pass 

 the actuating current through this 

 lamp in such a direction that the 

 covered or sheathed leg is the positive 

 leg, we find the effect existing as 

 before. A galvanometer connected 

 between the plate and positive ter- 

 minal of the lamp yields a strong 

 current, whilst if connected between 

 the negative terminal and the middle 

 plate there is no current at all. 

 Let us, however, reverse the current 

 through the lamp so that the shielded 

 or enclosed leg is now the negative 

 one, and the galvanometer is able to 

 detect no current, whether connected 

 in one way or the other. We establish, therefore, the conclusion that 

 it is the negative leg of the carbon loop which is the active agent in 

 the production of this " Edison effect," and that if it is enclosed in a 

 tube of either glass or metal, no current is found flowing in a galva- 

 nometer connected between the positive terminal of the lamp and this 

 middle collecting plate. 



Another experiment which confirms -this view is as follows: — 

 This lamp (see Fig. 7) hns a middle plate, which is provided with a 

 little mica flap or shutter on one side of it. When the lamp is held 

 upright the mica shield falls over and covers one side of the plate, 

 but when it is held in a horizontal position the mica shield falls away 

 from the front of the plate and exposes it. Using this lamp as before 

 we find that when the positive leg of the carbon loop is opposite to 

 the shielded face of the plate, we get the " Edison effect " as before 

 in any position of the lamp. Reversing the lamp current and making 

 that same leg the negative one, we find that when the lamp is so held 

 the metal plate is shielded by the interposition of the mica, and the 



Glow-lamp having negative leg of 

 carbon enclosed in glass tube T, 

 the " Edison effect" thereby being 

 annulled or greatly diminished. 



