328 Intelligence and Miscellaneous Articles. 



ON A SIMPLE METHOD OF INVESTIGATING THE THERMOELEC- 

 TRICITY AND PIEZOELECTRICITY OF CRYSTALS. BY A. KUNDT. 



The exact investigation of the thermoelectrical properties of 

 crystals is shown, more particularly by Hankel's comprehensive 

 researches, to be a very tedious and very difficult piece of work. 

 The determination of the electricity of each individual point of the 

 surface, by means of the electrometer, in order to arrive at the dis- 

 tribution over the whole surface, not only requires much time but 

 also a number of precautions. Besides thermoelectricity Hankel 

 distinguishes another mode of evoking electricity in crystals, 

 produced by rays, and more particularly those of low refrangi- 

 bility, and which he calls actino electricity : And, finally, pressure, 

 according to the discoveries of MM. Curie, produces electricity in 

 hemimorphic crystals, which they call piezoelectricity. The exact 

 investigation of actino- and of piezoelectricity presents in general 

 the same difficulties as those of thermoelectricity. 



In order to get a mean of investigating crystals in all directions, 

 it is desirable to have a method which enables us to demonstrate 

 the distribution of electricity over the whole system, even if the 

 method did not admit of quantitative measurements. I have found 

 such a method, which in simplicity and convenience of working 

 leaves nothing to be desired. The method is that used by Lichten- 

 berg to show the distribution of positive and negative electricity on 

 an insulator to which electricity has passed. 



In the moment in which the electrical distribution, produced in 

 a crystal by change of temperature, or pressure, is to be examined, 

 it is dusted with a mixture of red lead and sulphur which has been 

 sifted through fine cotton. By this means the powdered sulphur 

 becomes negative and the red lead positive : and, as in Lichten- 

 berg's figures, the negative sulphur deposits on those parts of 

 the surface which are positive and the red lead on the negative. 

 The distribution of the two powders then gives a very striking 

 image of the electrical distribution on the surface. 



The method described is so simple, and in fact so obvious, as to 

 make it improbable that it had not already been tried. I could not, 

 however, find in the literature in question any indication of it. I 

 have first only tried the method with a small number of substances, 

 namely quartz, tourmaline, and arragonite. Whether also the 

 thermoelectricity of those crystals in which it is exhibited in but 

 small degree can also be investigated by it I must leave undecided. 

 The experiments on the substances mentioned show its applica- 

 bility ; and I have since found that boracite, sugar, and tartaric 

 acid may be readily investigated by the method in question. 



The paper is accompanied by a chromolithographic plate showing 

 the figures seen in various crystals. — Berichte der Akad. der Wis- 

 senschaften zu Berlin, April 1883. 



