[ 146 ] 

 XX. Intelligence and Miscellaneous Articles. 



CHANGES OF VOLUMES OF DIELECTRICS. BY D. BOS. 



rPHE first of the three chapters into which the work is divided 

 -*- treats of the expansion observed by Duter, which hollow 

 glass tubes and bulbs show when they are charged as Leyden jars. 

 The explanation of this phenomenon is sought for by Kortevveg 

 in the electrical pressure upon the sides of the flask. The author 

 finds a sufficiently close agreement between the measurements 

 made by Quincke and the calculations of Korteweg, if the magni- 

 tudes occurring in the formula — the dielectric constant, the differ- 

 ence of potential of the coatings, the coefficient of elasticity, and the 

 thickness of the glass — are determined with sufficient accuracy. 



The experiments of Julius and Korteweg with caoutchouc tubes, 

 which show the influence of the coefficient of elasticity by a far 

 greater expansion than glass tubes of the same thickness, are given 

 as a strong reason for the opinion that the electrical pressure is the 

 chief cause of the expansion. 



A small part is ascribed to the heating of the dielectric. 



In the second chapter the forces which occur during a charge of 

 the dielectric are calculated. It is presupposed that with a relative 

 contraction (g) of the dielectric in the direction of the lines of 

 force, the dielectric constant h changes into k(l-\-4ag), and when 

 the contraction is at right angles to the lines of force, into 

 Ml + 4{3g). 



If F is the electromotive force at work in the dielectric, we get 

 in the direction of the lines of force a tension (k/87r + a/2)'F 2 on the 

 unit of surface, and at right angles to the lines of force a pressure 



(&/8rr-/3/2)F 2 . 



The author gives the developments of these expressions given by 

 Helmholtz and by Lorberg, and compares the results with the ex- 

 periments of Quincke on the attraction in the direction of the lines 

 of force, the repulsion at right angles thereto, and the change of 

 the dielectric constant with the magnitude of the electric forces. 



It is laid down that the measurements have not yet attained 

 such great accuracy as will permit of a and /5 being determined. 



The third chapter discusses Quincke's observations of the ex- 

 pansions and contractions of dielectric liquids contained in a kind 

 of voltameter whose plates are connected with the poles of a 

 charged Leyden jar. 



In agreement with Rontgen the author ascribes these changes of 

 Tolume to a heating of the liquid, and the extremely small con- 

 traction of water at 0° to the very small negative coefficients of 

 expansion, and to the great specific heat of water. 



With an apparatus founded on Quincke's the author finds an 

 expansion with rape oil, and also with almond oil, where Quincke 

 found a contraction. By means of a thermometer it was further 

 shown that on charging heat is produced between the plates which 



