152 



STATIC ELECTRICITY 



Then more or less straightening occurs and we get something of the 

 arrangement represented in Fig. 99, which shows that one of the 

 external connecting lines disappears, there is one more internal 

 connecting line, and there is one extruded line passing from end to 

 end of the crystal, not through the chains of molecules, and not 



FIG. 



even necessarily through the crystal. At each end of this line 

 there will be an element of what was previously the masking 

 charge, but is now what we may term free charge, and the con- 

 dition is represented in Fig. 100. This model would explain pyro- 

 electricity as due merely to molecular separations, and we ought 



FIG. 99. 



IS 



produced 

 K's found. 



to get a similar effect if the molecular separation 



by tension, and, as we shall see, this is just what the Curie: 



But we may imagine another molecular model in which pyro- 

 electric effects should occur if rise of temperature produces separation 

 of the constituent charges in each molecule without extension as a 

 whole. It will easily be seen that then the internal lines of force 

 bulge out and finally extrude a line of force running in the opposite 



-t- 



FIG. 100. 



direction to that in Fig. 100, and finally we have one more external 

 line and one less internal line, as represented in Fig. 101. In this 

 case the charges developed at the ends will be opposite to those 

 developed in the last case. It is possible that both effects occur. 



We may describe the electrical condition within the crystal on 

 this molecular chain theory by the number of tubes of strain 

 issuing per square centimetre from the end of an axis. We may 

 term this the " Intensity of Electrification " along that axis, and it 



