SOME SPECIAL MUTUAL CHANGES 



85 



the stress between it and the earth, for then the other stress 

 might be unnoticed. 



58. Construction of a Galvanometer. The stress existing 

 between a coil of wire forming part of an electric circuit and 

 a magnet is made use of in the instrument called a galvano- 

 meter. The simplest form of galvanometer consists of a 

 number of turns of copper wire, which is insulated by a silk or 

 cotton covering, wrapped round a wooden bobbin and sup- 

 ported vertically. The ends of the wire are connected by two 

 binding screws, by means of which the galvanometer can be 



Fig. 28. EEPRESENTS AN INSTRUMENT FOR SHOWING THE ACTION, UPON A MAGNET. 

 OF COILS FORMING PART OF AN ELECTRIC CIRCUIT. - 



A = magnetic needle with scale. 



B^B! = wooden bobbins, each with two sets of coils and binding-screws S to corre- 

 spond, one set of coils containing twice as many turns as the other. 



B 2 , B 2 = wooden bobbins with two coils like B! and B n but the diameters are just half 

 those of B! and B, and their supports enable them to be placed in the same plane as the 

 larger bobbins. * 



G = groove in which the four bobbins may slide, so as to vary their position with 

 regard to the magnetic needle. 



made to form part of an electric circuit. At the centre of this 

 coil of wire is fixed a glass- covered box, containing a magnet 

 suspended at its centre by a silk fibre. Underneath the magnet 

 is placed a graduated circle, by which we may read the angle 

 through which the magnet is deflected. 



Since the magnet will always be turned in the same direc- 

 tion by the mutual action between it and the earth, the gal- 

 vanometer is always placed so that its coil lies in this plane. 

 Any movement of the magnet must then be due to the stress 

 caused by the electric circuit. This stress will be found to 



