Electro-thermal Effect in Tourmaline. 223 



As regards the interpretation of the observations, I should 

 like to offer the following remarks : — There seems to me to 

 be no doubt that we really have to deal here with the electro- 

 thermal effect sought for. In support of this view we have 

 in the first place the regularity with which the effect changes 

 its sign with a reversal of the field. In none of the observa- 

 tions has this change of sign failed to take place. Further, 

 where no effects were to be expected, none occurred, or only 

 insignificant and irregular ones. Lastly, the magnitudes are, 

 for fields of approximately equal intensity, equal. Three 

 more extended series of observations, which were compara- 

 tively free from disturbance — l. e. from considerable changes 

 in the zero — gave the mean values 4*3, 4*5, and 4*1 scale- 

 divisions (2 mm. ones). 



The slow falling-off in the deflexion which takes place with 

 a constant P.D. between the condenser-plates is evidently 

 to be accounted for in the following way, which is also in 

 accord with other considerations. With the establishment of 

 an electric field there occur the temperature changes in the 

 two pairs of plates, there being a rise of temperature in one 

 pair and a fall in the other. The junctions begin to respond 

 to the temperature changes in their surroundings, and the 

 galvanometer gives a deflexion. But the electrical conduc- 

 tion along the tourmaline surface comes into play the instant 

 the field is established, and by the formation of surface 

 charges neutralizes the electrical action of the field on the 

 tourmaline. In proportion to the amount of this action, 

 however, the temperature-change will, of course, be reversed. 

 The greater part of the actually occurring temperature- 

 equalization is thus probably brought about by a decrease of 

 electrical excitation rather than by radiation or conduction 

 (although the metallic connexion of the two pairs of plates 

 by the thermopile might produce a perceptible effect) . 



As regards the amount of the observed electric change of 

 temperature, it may be remarked, in conclusion, that since 

 the d'Arsonval galvanometer was damped so as to be aperiodic, 

 it will be sufficient for the purpose in hand to multiply its 

 deflexions by half the value of the constant given above for 

 the degree value of a scale-division. This gives, in the ex- 

 ample considered, a temperature change of 0*74 x 10~ 3 deg. C. 

 for each pair of plates, a number which, so far as the order 

 of magnitude is concerned, is in sufficiently good agreement 

 with the value demanded by theory. 



Jena, February 20, 1902. 



