CHAP, ii.] ELECTRICITY AND MAGNETISM. 



97 



magnetic needle which is already attracted into a different 

 direction by a force of known intensity ; (4) by balanc- 

 ing it against the force of gravity as brought into play 

 in attempting to deflect a magnet hung by two parallel 

 strings (called the bifilar suspension), for these strings 

 cannot be twisted out of their parallel position without 

 raising the centre of gravity of the magnet. The jirst 

 three of these methods must be further explained. 



119. The Torsion Balance. Coulomb also applied 

 the Torsion Balance to the measurement of magnetic 



Fig. 55- 



forces. The main principles of this instrument (as used 

 to measure electrostatic forces of repulsion) were de- 

 scribed on p. 15. Fig. 55 shows how it is arranged for 



