MISCELLANEOUS PHENOMENA. 317 



disconnected from the voltaic cell and connected to a galvanometer 

 and the difference in temperature of the two sets of junctions is 

 shown by the deflection of the galvanometer. 



The Thomson effect. When a liquid, like water, flows along 

 a pipe which is not at a uniform temperature, the liquid always 

 absorbs heat from the pipe at each point when it flows in the direc- 

 tion of increasing temperature of pipe, and the liquid always gives 

 out heat to the pipe at each point when it flows in the direction of 

 decreasing temperature. Lord Kelvin (then Sir William Thom- 

 son) discovered in 185 1 that an electric current may either absorb 

 or give out heat at each point in a wire when the temperature of 

 the wire is not uniform. If the electric current absorbs heat at each 

 point of a wire when it flows along a wire in the direction of in- 

 creasing temperature, the Thomson effect is considered to be posi- 

 tive. If the electric current gives out heat at each point when it 

 flows in the direction of increasing temperature, the Thomson 

 effect is considered to be negative. 



19. Pyro-electricity.* A peculiar property of a crystal of 

 tourmaline after its temperature had been increased' or decreased 

 was noted by Daumius in 1 707. The crystal had the property of 

 attracting small particles of ashes. Aepinus in 1756 recognized 

 this property of a tourmaline crystal as an electrical phe- 

 nomenon, and he was able to show that the two ends of a 

 tourmaline crystal become oppositely charged when the tempera- 

 ture of the crystal is changed. Very extensive experimental 

 studies of the production of electric charges on the surface of 

 crystals by changes of temperature were carried out by Hankel, 

 beginning in 1839. Hankel found that the property of becoming 

 charged by a change of temperature is common to all crystals, 

 although hemihedral crystal forms show the effect more strik- 

 ingly. A method for demonstrating this so-called pyro-electric 

 property of crystals is to place a mixture of finely-powdered sul- 

 phur and red lead in a fine cotton sieve and dust it upon the 

 crystal after the temperature of the crystal has been changed. 



* See Wiedemann, Die Lehre von der Elektricitdt, Vol. II, pages 316-340. 



>PERTY OF ELECTRICAL LABORATORY, ; 



MOU'.-iV OK APfUED 



