120 



Prof. J. A. Fleming. Electric Discharge [Jan. 9, 



but if measured by a galvanometer the potential difference of the 

 plate and the positive electrodes of the lamp is something less than 

 that of the working lamp electrodes. 



(3.) This absolute equality of potential between the negative elec- 

 trode of the lamp and the insulated plate only exists when the carbon 

 filament is in a state of vivid incandescence, and when the insulated 

 plate is not more than an inch or so from the base of the negative leg. 

 When the lamp is at intermediate stages of incandescence, or the 

 plate is considerably removed from the base of the negative leg, then 

 the plate is not brought down quite to the same potential as the nega- 

 tive electrode. 



(4.) A galvanometer connected between the insulated plate and the 

 positive electrode of the lamp shows a current increasing from zero to 

 four or five milliamperes, as the carbon is raised to its state of com- 

 mercial incandescence. There is not any current greater than 

 0*0001 of a milliampere between the plate and negative electrode 

 when the lamp has a good vacuum. 



(5.) If the lamp has a bad vacuum this inequality is destroyed, and 

 a sensitive galvanometer shows a current flowing through it when 

 connected between the middle plate and either the positive or nega- 

 tive electrode. 



(6.) When the lamp is actuated by an alternating current a con- 

 tinuous current is found flowing through a galvanometer, connected 

 between the insulated plate and either terminal of the lamp. The 

 direction of the current through the galvanometer is such as to show 

 that negative electricity is flowing from the plate through the gal- 

 vanometer to the lamp terminal. This is also the case in (4) ; but, if 

 the lamp has a bad vacuum, then negative electricity flows from 

 the plate through the galvanometer to the positive terminal of the 

 lamp, and negative electricity flows to the plate through the gal- 

 vanometer from the negative terminal of the lamp. 



(7.) The same effects exist on a reduced scale when the incan- 

 descent conductor is a platinum wire instead of carbon filament. The 

 platinum wire has to be brought up very near to its point of fusion, 

 in order to detect the effect, but it is found that a current flows 

 between the positive electrode of a platinum wire Janip and a platinum 

 plate placed in the vacuum near to the negative end of that wire. 



(8.) The material of which £he plate is made is without influence. 

 Platinum, aluminium, and carbon have been indifferently ernployed. 



(9.) The active agent in producing this effect is the negative leg of 

 the carbon. If the negative leg of the carbon is covered up by 

 enclosing it in a glass tube this procedure entirely, or nearly entirely, 

 prevents the production of a current in a galvanometer connected 

 between the middle plate and the positive terminal of the lamp. 



(10.) It is a matter of indifference whether a glass or metal tube is 



