26 On the Pyroelectric Properties of Matter. 



mass or per unit of bulk. If the directions of x, y, z and 

 f , 77, f are such that P, Q, R, S, T, U are positive, then for 

 for iron and nickel, and for cobalt at temperatures above that 

 of its maximum inductive capacity, 



d? 



dQ 



dU 



dS 



dT 



dU 



dt' 



dt' 



dt' 



dt' 



dt 1 



dt 



are positive, and therefore the substance experiences a cooling 

 effect when it is moved in such a manner as to require work 

 to be done against magnetic force ; and the reverse is the case 

 for cobalt at ordinary temperatures.] 



28. Extracted from NichoVs 'Cyclopaedia of the Physical 

 Sciences' second edition, I860.— The most probable account 

 that can be given of the pyroelectric quality of dipolar crys- 

 tals is, that these bodies intrinsically possess the same kind of 

 bodily electro-polarization which Faraday, in his 'Experimental 

 Researches,' has clearly proved to be temporarily produced in 

 solid and liquid nonconductors, and that they possess this 

 property to different degrees at different temperatures. 



The inductive action exercised by this electro-polar state of 

 the substance, on the matter touching the body all round, in- 

 duces a superficial electrification which perfectly balances its 

 electric force on all points in the external matter ; but when 

 the crystal is broken in two across its electric axis, the two 

 parts exhibit as wholes contrary electrifications, not only by 

 the free electro-polarities on the fractured surfaces discovered 

 by Canton, but by the induced electrification on the old sur- 

 face, belonging to the old state of electric equilibrium, and 

 gradually lost by slow conduction, while a new superficial 

 distribution of electricity on each fragment is acquired which 

 ultimately masks all external symptoms of electric excitement. 

 When the temperature of the substance is changed, its electro- 

 polarization changes simultaneously, while the masking super- 

 ficial electrification follows the change only by slow degrees — 

 more or less slow according to the greater or less resistance 

 offered to electric conduction in the substance or along its 

 surface. 



[29. If the preceding explanation of pyroelectricity be 

 true, it must follow that a pyroelectric crystal moved about 

 in an electric field will experience cooling effects or heating 

 effects calculable by formula (22) of § 20, with the same no- 

 tation for the electric subject as that of § 27 for magnetism. 

 Thus the effects will be the same for a crystal at the same tem- 

 perature whatever be the electrification of its surface. Thus 

 it is remarkable that, in virtue of the wholly latent electric 



