THE VOLTAIC ARC. 23 



but it may be assumed that under the conditions of a good 

 light, this resistance may reach two or three thousand metres 

 of telegraphic wire, say of 20 to 30 ohms. The figures given 

 by different physicists are, however, far from being accordant ; 

 for while Preece, Schwendler, and others assign to the arc a 

 resistance of i to 3 ohms, Ayrton and Perry fix the figures 

 at from 20 to 25 ohms, at least with batteries ; but it appears 

 that with machines the resistance is less. 



An interesting fact has been observed by Le Roux, namely, 

 that if the voltaic arc is not formed in the cold when the 

 carbons are separated by a layer of air, however small, it 

 may be spontaneously developed from one carbon to another, 

 across a space attaining nearly 3 millimetres, and after the 

 current has been interrupted during a period which may last 

 for about h of a second. This explains why the alternately 

 reversed currents of certain magneto-electric machines are able 

 to yield a continuous light, although themselves discontinu- 

 ous, for the current in the Alliance machines is periodically 

 interrupted every one or two ten-thousandth part of a second. 



We have stated that the voltaic arc depends much upon 

 the medium in which it is formed. In a vacuum the length 

 of the arc can be considerably increased, and although there 

 is no combustion, there occurs a disintegration of the material 

 particles of the electrodes, which are carried in both directions 

 by the current, and even spread through the surrounding 

 space. When the arc is formed in different gases its appear- 

 ance varies but little ; there are scarcely any changes of 

 colour, but secondary chemical actions may occur, and then 

 the length and colour of the arc will be modified. Of course, 

 in gases incapable of supporting combustion the carbon 

 electrodes, although becoming disintegrated, will not burn, 

 but the brightness of the arc is diminished. 



The electric light has a great analogy with that of the 

 sun, but the former contains more of the chemical rays, a 

 circumstance which renders it rather hurtful to the sight. 

 Means of avoiding this inconvenience have indeed at various 



