Transmission of Electric Disturbances. 137 



Ann. xxi. p. 90) has observed this effect to a very striking 

 extent: at a pressure of '03 centim. of mercury it took 1100 

 elements of his battery to force the discharge through a tube 

 1 centim. in diameter, while 300 elements were sufficient to 

 force it between similar electrodes in a tube 11 centim. in 

 diameter. 



In the structure round the negative electrode, consisting 

 of the glow over the electrode, Crookes' dark space, and the 

 negative glow, which, according to our view, is only a stria 

 differing from other striae merely by the special arrangement 

 of the lines of force near the electrode, the chemical and other 

 changes produced by the discharge seem to be more compli- 

 cated than those which take place in the rest of the field : one 

 obvious reason for this is the presence of the metal, which 

 makes many chemical changes possible which could not take 

 place in strise where nothing but gas is present. This stria is 

 thus under special conditions, and may differ in size, fall of 

 potential, &c. from the remaining striae. 



Hittorf 's experiments show that there is an exceptionally 

 large change in potential close to the kathode. If we take the 

 preceding view of the spark-discharge this will explain the very 

 striking increase in the electromotive intensity necessary to 

 produce a spark across a layer of air when the thickness of 

 the layer is diminished beyond a certain amount. For let 

 us consider the case when the spark passes between two 

 infinite parallel planes ; the discharge, according to the view 

 enunciated above, goes first across the stria next the nega- 

 tive electrode, where the rise in potential is K, and then 

 across a number of other striae, the rise of potential across 

 each stria being w ; thus if n be the number of such striae, 

 Y the difference of potential between the plates when the 

 spark passes, 



V=K+ratf. 



If I is the distance between the plates, X the length of the 

 stria next the kathode, \ the length of the other striae, then 



I — X 



This may be written 



V=(K- £«)+£* • • • (1) 



V = K' + aZ. 



On this view the curve representing the relation between 

 difference of potential and spark-length is a straight line not 



