154 



STATIC ELECTRICITY 



When the vice was screwed up, a charge was shown in the 

 electrometer. On discharging this and then releasing the crystal 

 from pressure the polarity was reversed. 



A number of crystals were thus tested, and it was found that 

 always compression gave the same sign of charge at one end as 

 cooling, and extension the same sign as heating.* Working with 

 tourmaline and applying pressure by a loaded lever by which the 

 force could be measured, it was found that 



1. The positive and negative charges were equal. 



2. The change in charge de developed was proportional to the 



change in the total force dF and changed sign with dF. 



3. The charge was independent of the length. 



4. The charge was independent of the cro*s-section for a given 



total force. Hence de <x dF. 



We may use the first molecular model described on p. 151 to 

 give us an idea of a possible way in which the effect is produced) 



FIG. 102. 



the 



changes in 



dimensions beinj 



produced now by tension and 

 pressure instead of by heating and cooling. 



To find the amount of charge produced, an arrangement repre- 

 sented in Fig. 103 was adopted. A block of the crystal with its ends 

 lined with tinfoil was placed between insulating ebonite plates, and a 

 known force F could be applied by a lever, not shown. The upper 

 tinfoil was connected to one plate of a small condenser of known 

 capacity C, and also to one pair of quadrants of an electrometer 

 QE. The lower tinfoil and the other plate of the condenser were 

 earthed. The other pair of quadrants of the electrometer was 

 charged by a DanielFs cell D so connected that the sign of the charge 

 was the same as that given by the crystal when a force F was put on 

 to it. This force was then so adjusted that the electrometer needle 

 came to the zero reading found when the two pairs of quadrants 

 were earthed. 



* The account of the expeiinieLts mace by J. and P. Curie is given in a seiies 

 of papers in CEuvres de Pierre Curie. 



