462 Mr. A. P. Chattock on an 



Arrange two sets of equally, oppositely, and permanently 

 charged spheres, A, B, C, (fig. 1) in a frictionless, 



Fig. 1, 



horizontal, non-conducting tube which just fits them ; so that 

 they stand in line, alternately -f and — along its length. 

 Their mutual attractions will at once bring them into contact 

 with each other ; and the resulting chain of spheres will be 

 electrically polarized — a sort of electrostatic magnet ; the 

 amount of free 4- or — electricity at either end being equal, 

 roughly speaking, to half that possessed by each of the 

 spheres. 



Now endow the spheres with a mutually repulsive tendency, 

 such as is possessed by the molecules of a body due to their 

 mechanical heat-motion. This will open out the chain length- 

 wise, but it will not do so uniformly. There will be a pair- 

 ing tendency among the spheres — A tending to pair with B, 

 C with D, and so on. This may result in the complete dis- 

 integration of the chain — the spheres flying off in pairs ; but 

 if the repulsive action is not powerful enough to effect this, it 

 will still give rise to a partial pairing ; with the result that 

 A will be nearer to B than B to C, and so on ; A not having 

 separated from B quite so far as B from C. This means, of 

 course, that of the whole number of lines of force emanating 

 from A, a greater proportion goes to B than before, and 

 therefore fewer remain to pass out into space and give rise to 

 external effects. In other words, the chain possesses less free 

 electricity at its ends than before ; and when the pairing is 

 complete it possesses none. 



The application of the above to pyro-electricity is obvious. 

 It is only necessary to suppose that, in the building-up of 

 the crystal, chains of alternately positively and negatively 

 electrified molecules occur parallel to its pyro-electric axis ; 

 and, provided no alteration in the formation takes place 

 during the process, a change of temperature from absolute 

 zero (where the molecules are in contact and the free charge 

 is a maximum) to evaporation-point (where they fly off in 

 pairs and the free charge has vanished) will give rise to an 

 alteration in the free charge at the end of each chain equal 



