and on the Pyro-Eleclricity of Glass. 



345 



Experiment 1. — Having heated and excited a glass tube 

 by means of a silk handkerchief*, I held it parallel to a 

 brass needle about three inches in length and one-fourth of 

 an inch in width, and placed on a pivot with a wide and 

 shallow cap and insulated stand ; the needle began immedi- 

 ately to revolve on its centre of motion, and continued for 

 fifty or sixty revolutions; it then began to oscillate, and after 

 about twenty rounds fixed at right angles to the tube. The 

 following diagram may give a more defined explanation : — 



A 



K 



Let ABCDEFGH represent a horizontal plane, through 

 which the needle or brass bar A E revolves on the fulcrum or 

 centre of motion m. When the excited glass tube is held par- 

 allel at K, the arm of the bar A E is powerfully attracted to 

 C, where it becomes like the glass tube, electro-positive ; con- 

 sequently it is now repelled to E with a force equal to that 

 of attraction. By this motion E has taken the place of A, and 

 is become, by giving out electricity, electro-negative, and is 

 therefore attracted. Thus continual revolutions are pro- 

 duced so long as the electric force, viz. attraction, repulsion, 

 and moment, are together equal to 180°; the velocity being in 

 a direct ratio to the force of attraction and repulsion, so that 



CE 



A : C : : E : C, &c., —^ = C. After some time the attraction 



and repulsion AC, EC decrease, and being together less 

 than 180°, oscillation takes place, say within the sector BmD, 

 continuing according to a decreasing series. Biot and Cou- 

 lomb have established the law, that " the loss of electricity is 

 equal in equal time," and equal to the force of attraction and 



• This experiment can be conveniently made with a warm excited tum- 

 bler or wine-glass ; thus | j "[" 



