SCIENCE AND PRACTICE. 



237 



centre with a force which is directly proportional to the 

 superficial contents of the circle, and inversely to third power 

 of the distance of the element from the periphery. 



16. Tangent Galvanometer. When a magnetic needle is 

 deflected by a ring in which a current is circulating, the force 

 of the deflection is the resultant of the attraction and repul- 

 sion of the ring upon all the magnetic elements composing 

 the needle ; but as these attractions and repulsions depend 

 upon the position of the needle with respect to the ring, and 

 alter with it, when the former is turned from its normal 

 position the resultant is evidently no longer the same. To 

 reduce the difference, however, to an amount so minute as to 

 be neglected, the diameter of the ring must be made so great 

 in comparison with the length of the needle that the distance 

 of each magnetic element from the periphery, in whatever 

 position the needle may stand to the 

 axis of the ring, may be said to be 

 the same. 



When the needle n s, Fig. 122, is 

 at rest, and the ring surrounding it 

 is in the plane of the magnetic meri- 

 dian, any current in the ring which 

 tends to deflect it from its position of 

 rest acts at right angles to the direc- 

 tion of the horizontal component of 

 the earth's magnetism, and its force 

 may be represented by a line through 

 the pole of the needle at right 



angles to magnetic north and south. Such a current will 

 deflect the needle, which will take up a position, n s', making 

 an angle, a, with its former position. Suppose the whole 

 magnetism of the needle to be collected in the pole n, and 

 this to be acted upon, in the direction n s by the magnetism 

 of the earth, and in a direction at right angles to n s by the 

 magnetism of the ring ; when the pole n takes its place at 

 n', these forces are balanced and equivalent to a force acting 

 in the direction n' s 1 . Let this force be decomposed into the 

 forces a n and an' at right angles to each other ; each of 



