PYROELECTRICITY AND PIEZOELECTRICITY 151 



positive and negative at the two ends, and giving out the same 

 number of tubes of strain as those actually issuing. But through 

 the positive and negative ions which always exist to some small 

 extent in the air, through conducting films on the crystal and 

 sometimes by conduction in the body of the crystal itself, charges 

 will in time gather on the surfaces at the ends of the axis, negative 

 on the end from which tubes of strain issue, and positive on the 

 other end into which they enter, entirely masking the existence of 

 the strain so that there is no external field. This is the ordinary 

 neutral condition of a pyroelectric crystal. 



We have now to suppose that the electric strain passing forward 

 from molecule to molecule depends on the temperature and that 

 it. alters nearly in proportion to the change of temperature. Then 

 when the temperature changes there will be an unbalanced strain 

 proportional to 'the change, producing the same effect as a positive 

 charge at one end and a negative charge at the other until con- 

 duction again does its work in bringing up masking charges. The 

 apparent charges are proportional evidently to the cross-section 

 and independent of the length. 



We may picture a molecular model which might give the effect, 

 and though it suffers from the common defect of such models in 

 being statical, whereas in reality everything must be in motion, it 

 will at least serve as a working hypothesis. We suppose the 

 molecules to be doublets set with their axes all parallel to a given 

 direction and with the positive part of each to the right, as in Fig. 

 97, and we suppose that there is some connection between the 

 positive of each molecule and the negative in the next. For 

 instance, in Fig. 97 there are five internal lines of force in each 



FIG. 97. 



molecule, and three external lines connecting it to the next on each 

 side, and these lines run in opposite directions from the same 

 charge. The external lines to the extreme molecule on the left 

 begin on the positive masking charge, and those from the extreme 

 molecule on the right end on the negative masking charge. 



Let us further suppose that through a rise in temperature the 

 molecules are more separated. Then the external lines of force 

 will bulge out, as it were, and this bulging out may take place to 

 such an extent that one between each pair of molecules may touch 

 one between each of the next pairs on either side, as represented 

 in Fig. 98, where only the external lines concerned are drawn. 

 At the points of contact P and Q the strains are in opposite 

 directions in the two coalescing lines and neutralise each other. 



