CHAP, i.) ELECTRICITY AND MAGNETISM. 



15. The Torsion Balance. Although more pro- 

 perly an Electrometer than a mere Electroscope, it 

 will be most convenient to describe here the instrument 

 known as the Torsion 

 Balance. (Fig. 9.) This 

 instrument serves to 

 measure the force of the 

 repulsion between two 

 similarly electrified 

 bodies, by balancing the 

 force of this repulsion 

 against the force exerted 

 by a fine wire in untwist- 

 ing itself after it has been 

 twisted. The torsion 

 balance consists of a 

 light arm or lever of 

 shellac suspended within 

 a cylindrical glass case 



by means of a fine silver wire. At one end this lever is 

 furnished with a gilt pith-ball, n. The upper end of the 

 silver wire is fastened to a brass top, upon which a circle, 

 divided into degrees, is cut. This top can be turned 

 round in the tube which supports it, and is known as the 

 torsion-head. Through an aperture in the cover there 

 can be introduced a second gilt pith -ball m, fixed to ' 

 the end of a vertical glass rod a. Round the glass case, 

 at the level of the pith-balls, a circle is drawn, and 

 divided also into degrees. 



In using the torsion balance to measure the amount 

 of a charge of electricity, the following method is 

 adopted : First, the torsion-head is turned round until 

 the two pith -balls in and n just touch one another. 

 Then the glass rod a is taken out, and the charge of 

 electricity to be measured is imparted to the ball m, 

 which is then replaced in the balance. As soon as m 

 and n touch one another, part of the charge passes from 



