Electrolytic Theory of Dielectrics. 463 



in amount to about half the charge possessed by each mole- 

 cule. But there is no necessity for the chains to be opened 

 out by heat in order to cause pairing of the molecules. 

 Tension along their lengths will give rise to the same effect, 

 while compression will correspond with cooling ; the result of 

 either being what is known as piezo-electricity. Such an 

 agreement of heating with tension and of cooling with com- 

 pression appears to exist*. It is true that it is not invari- 

 able ; but there seems to be no great difficulty in supposing 

 that in those cases where the rule is reversed, disturbing effects 

 may exist due to some action on the chains of the molecules on 

 either side of them ; for while both heating and tension affect 

 the chains similarly, they produce opposite effects in planes at 

 right angles to them — heating decreasing, wdiile tension in- 

 creases the proximity of the molecules therein. The theory 

 does not therefore necessitate the above agreement — it only 

 renders it likely ; but the calculations which follow can only 

 be made when the agreement occurs. 



The point at which the free piezo-electric charge is a 

 maximum, and which corresponds with the absolute zero of 

 temperature in the case of pyro-electricity, occurs at a pres- 

 sure which is sufficient to bring the molecules into contact. 

 As this pressure is reduced the pairing tendency comes more 

 and more into play, until, after changing from compression 

 to tension, the breaking-point is reached. Now the break 

 occurs when the cohesion of the weakest place in the test- 

 piece is overcome by the pull ; and, on the above view r , the 

 weakest place will be found between the molecules which 

 are farthest apart in the chains (flaws being supposed absent), 

 i, e. between B and C, D and E, &c. (fig. 1). On the other 

 hand, from the fact that when a crystal breaks it does so 

 under a definite tension, it follows that the pairing is not com- 

 plete at the point of rupture. Hence all that can be said is 

 that the pairing is on the way towards completion, and, in the 

 light of what follows, that an appreciable amount of it has 

 been already accomplished when this point is reached. A 

 change of stress, then, along the piezo-electric axis of a 

 crystal, from a compression which brings the molecules into 

 contact to a tension which breaks the crystal, will alter the 

 free charge at the end of each molecular chain by an amount 

 which is less than, but probably of the same order of magni- 

 tude as, half the charge possessed by each molecule. 



* Wiedemann's Electricitat, ii. p. 342. 

 2 K2 



