164 



ELEMENTARY LESSONS ON [CHAP. in. 



by setting the coil in the magnetic meridian, the small 

 needle lying then in the plane of the coil. One essential 

 feature of this arrangement is, that while the coil is very 

 large, the needle is relatively very small. The " field " 



due to a current passing round the circle is very uniform 

 at and near the centre, and the lines of force are there 

 truly normal to the plane of the coil. 1 This is not true 

 of other parts of the space inside the ring, the force 

 being neither uniform nor normal in direction, except in 

 the plane of the coil and at its centre. The needle being 



1 In order to ensure uniformity of field, Gaugain proposed to hang the 

 needle .at a point on the axis of the coil distant from its centre by a distance 

 equal to half tha radius of the coils. Helmholtz's arrangement of two 

 parallel coils, symmetrically set on either side of the needle, is better ; and a 

 three-coil galvanometer having the central coil larger than the others, so that 

 all three may lie in the surface of a sphere having the small needle at its j 

 centre, is the best arrangement of all for ensuring that the field at the centre 

 is uniform. 



