Foundations of the Universe 273 



rush violently and explosively from one body to another, as in 

 the electric spark or the occasional flash from an electric tram or 

 train. The grandest and most spectacular display of this phenom- 

 enon is the thunder-storm. As we saw earlier, a portentous fur- 

 nace like the sun is constantly pouring floods of electrons from 

 its atoms into space. The earth intercepts great numbers of these 

 electrons. In the upper regions of the air the stream of solar 

 electrons has the effect of separating positively-electrified atoms 

 from negatively-electrified ones, and the water-vapour, which is 

 constantly rising from the surface of the sea, gathers more freely 

 round the positively-electrified atoms, and brings them down, as 

 rain, to the earth. Thus the upper air loses a proportion of posi- 

 tive electricity, or becomes "negatively electrified." In the thun- 

 derstorm we get both kinds of clouds some with large excesses 

 of electrons, and some deficient in electrons and the tension 

 grows until at last it is relieved by a sudden and violent discharge 

 of electrons from one cloud to another or to the earth an elec- 

 tric spark on a prodigious scale. 



Magnetism 



We have seen that an electric current is really a flow of elec- 

 trons. Now an electric current exhibits a magnetic effect. The 

 surrounding space is endowed with energy which we call electro- 

 magnetic energy. A piece of magnetised iron attracting other 

 pieces of iron to it is the popular idea of a magnet. If we arrange 

 a wire to pass vertically through a piece of cardboard and then 

 sprinkle iron filings on the cardboard we shall find that, on pass- 

 ing an electric current through the wire, the iron filings arrange 

 themselves in circles round it. The magnetic force, due to the 

 electric current, seems to exist in circles round the wire, an ether 

 disturbance being set up. Even a single electron, when in move- 

 ment, creates a magnetic "field," as it is called, round its path. 

 There is no movement of electrons without this attendant field 



VOL. I 18 



