INTENSITY OF MAGNETISATION. 623 



But the product of the induced discharge by the resistance R 

 of a circuit (515) is equal to the work W which corresponds to 

 unit current. We have then 



If the circuit is connected only with a ballistic galvanometer and 

 the magnet is suddenly carried to a great distance, the corresponding 

 discharge q will give the product MG cos 9 that is to say, the pro- 

 duct of the constant G of the frame by the component M x = M cos 9 

 of the magnetic moment parallel to the field. 



If the magnet were first parallel to the field, a rotation of 90 

 would give the product MG, and a reversal end for end the value 

 2MG. 



We may use in this way either a long cylindrical coil or a 

 spherical coil (497), or any given system of frames with a uniform 

 field (750 and 751). 



Measuring by this method the components M^, M^, and M z 

 of the magnetic moment of the body in respect of three rectangular 

 axes, we shall arrive at the resultant moment 



and the angles a, /?, 7 of the direction of this moment with the axes 

 will be defined by the equations 



cos a cos/3 cosy i 



1196. INTENSITY OF MAGNETISATION. The quotient of the 

 magnetic moment of a body by its volume gives the mean in- 

 tensity of magnetisation. 



From the experiments of Gauss,* for instance, the magnetic 

 moment of the earth in C.G.S. units is 



0-33092 R 3 = 8'55.io 25 . 



In other words, the mean value of the horizontal component 

 of the terrestrial field at the magnetic equator being about 0-33092, 



* GAUSS. Allgem. Theorie des Erdmagn. (Euvres, Vol. V., p. 164. 



