382 



WELLS'S NATURAL PHILOSOPHY. 



Fig. 320. 



consists of a semicircle of varnished paper, or ivory, fixed upon a vertical 

 rod. From the center of the semicircle a light pith-ball is suspended, and 

 the number of degrees through which the ball is attracted or repelled by any 

 body brought in proximity to it, indicates in a degree the active quantity of 

 electricity present. No very accurate results, however, can be obtained with 

 this apparatus ; and for accurate investigation, instruments of more ingenioua 

 aud complicated construction are used. 



The electrometer usually employed for measuring with 



great accuracy small quantities of electricity, is that of 



Coulomb's, usually called the Torsion Balance. 



„ , . ., The construction of this instrument is as follows : — A needle, 



Explain the . _' 



construction of or stick of shell-lac, bearing upon one end a gilded pith-ball, is 

 STiance!''"^'*"*" suspended by a fiber of silk within a glass vessel — the needle 

 being so balanced, that it is free to turn horizontally around 

 the point of suspension in every direction. When the pith-ball is electrified 

 by induction, the repellent force causes the needle to turn round, and this 

 produces a degree of torsion, or twist in the fiber which suspends it ; and the 

 tendency of the fiber to untwist, or return to its original position, measures 

 the force which turns the needle. 

 Within the glass vessel, which is cylin- 

 drical, a graduated circle is placed, 

 which measures the angle through 

 which the needle is deflected. In the 

 cover of the vessel an aperture is made, 

 through which the electrified body may 

 be introduced, whose force it is desired 

 to indicate and measure by the ap- 

 paratus. Fig. 320 represents the con- 

 struction and appearance of the torsion 

 balance. 



By means of the 



torsion balance, 



Coulomb proved 



that the law of 

 electrical attraction and re- 

 pulsion, as influenced by dis- 

 tance, is the same as the law 

 of gravitation ; that is, the force varies inversely as the 

 square of the distance. 



What is .a Ley. 753. Thc Lcydcn Jar is a glass vessel used 

 den Jar? ^^^ ^|^g purposc of accumuLiting electricity de- 

 rived from electrically excited surfaces. 



WTiat import- 

 ant law of 

 electricity has 

 been proved by 

 the trtrsion bal- 

 ance? 



