52 THE NEW KNOWLEDGE. 



taining an insulated metal plate, A, which is in connection 

 with an electrometer; underneath this plate there is a 

 fine metallic wire, which can easily be raised to incandes- 

 cence by an electric current passing through it from C to 

 D. Taking, first, the case in which the air within the ves- 

 sel is at the ordinary pressure of the atmosphere, it will 

 be observed that, as the wire grows hotter and hotter, un- 

 til it just begins to glow, the metal plate receives a charge 

 of electricity, and, furthermore, that this charge is posi- 

 tive. It will next be observed that this charge steadily grows, 

 until the wire is at a yellow heat, when it is at a maxi- 

 mum ; and that, after passing this stage, it diminishes rap- 

 idly with the increasing temperature of the wire, until, 

 when the wire reaches a bright white heat, the charge re- 

 ceived by the plate is very small. 



So far, we learn that not only may a hot wire as well 

 as a candle flame constitute an electric battery, but that 

 these gaseous ions, which pass from the wire to the plate, 

 need not necessarily convey both kinds of electricity, but 

 may convey one kind only, that kind being positive. At 

 this stage of the experiment, then, and while the wire is 

 still hot, pump out the air within the vessel, which, it must 

 be remembered, has so far been at the pressure of the 

 atmosphere. At first, little change will be noticed in the 

 positive charge upon the metal plate; but as the exhaustion 

 proceeds, at a certain point, depending upon the tempera- 

 ture of the wire, the sign of the electricity upon the plate 

 changes. It is now negative, not positive; and when almost 

 all the air but not quite all has been pumped out of the 

 vessel, this negative charge may reach a high value. 



We learn by this that the gaseous ions comprise particles 

 of two kinds, one carrying positive electricity and the other 

 negative; and we are now in a position to understand why 



