486 PARTICULAR CASES. 



503. MEASUREMENT OF CURRENTS. GALVANOMETERS. The 

 strength of currents is usually measured by the electromagnetic or 

 electrodynamic actions which they exert, and the instruments which 

 are used for this purpose are called galvanometers, or electrodyna- 

 mometers, according as they depend on one or the other of the two 

 actions. 



A galvanometer consists of a magnetised needle, or of any 

 magnetic system on which a conductor traversed by a current is 

 made to act; the effect produced is measured by means of an 

 antagonistic force, such as the torsion of a metal wire, or of a bifilar 

 suspension, or by the action of an external magnetic field. 



Let us consider the simple case of a horizontal magnetic needle 

 suspended by a wire without appreciable torsion, and placed in the 

 centre of a frame on which is coiled a wire forming a series of 

 parallel turns. 



If the turns are parallel to the magnetic meridian, and they are 

 traversed by a current, they produce a magnetic field, the strength of 

 which is proportional to the strength of the current, and which 

 may be represented by GI. The horizontal component of the 

 terrestrial field at this point being H, the horizontal component of 

 the field is ^/G' 2 ! 2 + H 2 , and its direction makes an angle 8 with the 



C*T 

 magnetic meridian, the tangent of which is equal to . 



An infinitely small needle placed at this point, and which at 

 first was in equilibrium in the plane of the needle, will be deflected 

 through an angle <5, and from it we may deduce the strength of the 

 current by the expression 



TT 



This formula is only exact provided the magnetic field is uniform 

 throughout the whole space which the needle occupies. When the 

 needle has a length which is considerable in reference to the 

 dimensions of the frame, the intensity of the field is not constant, 

 and the formula for the deflection is less simple. In that case, by 

 an empirical graduation, we could determine the ratio which exists 

 between the strength of the current and the deflection produced. 



The magnetic moment of the needle has no influence on its 

 position of equilibrium ; it has no other effect than that of modifying 

 the strength of the forces, and therefore the duration of the 

 oscillations of the needle. 



