150 STATIC ELECTRICITY 



is immersed in liquid air and is then lifted out and dusted with red 

 lead and sulphur before it has time to condense ice on its surface, 

 it shows the colours excellently. 



Gaugain' s researches. Gaugain* employed an air bath in 

 which a tourmaline crystal was suspended. One end of the crystal was 

 earthed and the other end was connected to a self-discharging gold- 

 leaf electroscope, and the total charge developed was measured by 

 the number of discharges of the gold leaf. He found that 



1. The charge is proportional to the change of temperature from 



neutrality. 



2. With different tourmalines of the same kind the charge is 



independent of the length and is proportional to the cross- 

 section. 



Rlecke's researches. Rieckef made important contribu- 

 tions to our knowledge of the subject. He investigated the loss of 

 charge through leakage and showed that it was very much 

 diminished if the crystal was suspended in a dry vacuum. He 

 showed that if $ is the change of temperature from neutrality, the 

 charge is not exactly proportional to $, but may be represented 

 more nearly by 



E = S + b&. 



He worked in a manner similar to that of Gaugain, and graduating 

 the electrometer by known charges, he was able to determine 

 the charges developed in electrostatic units. Thus for a certain 

 tourmaline which gave the largest effect he found that 180 E.S. 

 units per square centimetre cross-section were developed per 100 C. 

 rise of temperature. 



Lord Kelvin's theory of pyroelectricity. To Lord Kelvin J 

 we owe the theory of the subject which is generally accepted. Let 

 us take the simple case of a crystal like tourmaline with one 

 pyroelectric axis. We suppose that each molecule of the crystal 

 is an electric doublet with lines of force or strain passing between 

 the pair and into and out of the ends. The molecules set in the 

 crystal with their electric axis parallel to a certain direction, and 

 there will therefore be a definite amount of strain passing from 

 each positive element of a doublet to the negative element of the 

 doublet next to it. The crystal as a whole will be the seat of 

 electric strain along the direction of the axis. The constitution 

 corresponds to that of a saturated permanent magnet on the 

 molecular theory of magnetism. The electric strain will manifest 

 itself almost entirely at the ends of the crystal if the doublets are 

 near enough together, and the crystal will produce a field outside 

 equal to that which would be produced by charges respectively 



* Ann. de Chim. [3], 57, p. 5 (1859). 



f Wied. Ann., 28, p. 43 (1886) ; 31, p. 889 (1887) ; id. p. 902 ; 40, p. 264 (1890) ; 

 id. p. 305. 



J Nicol's Cyclopaedia, 2nd ed. (I860), or Math, and Phys. Papers, vol. i. p. 315. 

 The fullest account is in the Baltimore Lectures, p. 559. 



