276 



A. TANAKADATK 



In sncli a state of equilibrium, if a pole of iinother niaî^net, or a 

 wire conducting a current be brought near, the first magnet wilM^e 

 pulled or pushed as the case may be, so as to rotate round the fixed 

 axis; the only necessary condition being that the forces acting on the 

 two poles of the magnet shall be unequal, as will generally be the 

 case in the neighbourhood of a straight conductor. The circum- 

 stances of rotation will depend upon the strengths of the magnet and 

 of the current respectively. 



Any small portion of a closed electric circuit may be looked upon 

 as the edge of a large magnetic shell. Now, if two opposite poles of 

 a small magnet be placed close to the edge of such a shell at equal dis- 

 tances from the edge, the electro-magnetic force acting on the magnet 

 will be constant, provided the line joining the two poles always passes 

 through the edge of the shell, no matter how the other portions of 

 the shell may lie with regard to the magnet. If four poles in rigid 

 connection be placed about such an edge, it will be possible to find such 

 an arrangement of the poles that the force acting on the system of the 

 ])oles will be sensibly constant when the edge is within a certain 

 portion of space between the four poles. 



Thus it becomes possible to construct an instrument which will 

 measure the strength of such a shell without breaking its continuity, 

 and that independently of the uniform field in which the shell may be 

 situated. That is, an instrument which will measure the current 

 without breaking the circuit and which can be used in any position 

 and in any uniform field. This is the idea upon which the apparatus 

 to be described is constructed. It is indeed simply a modified form 

 of an astatic galvanometer. 



The following is one particular form of the apparatus : — Four 

 small bar magnets are fixed symmetrically at the four corners of a, 

 thin rectangular plate of wood, which can I'otate as a whole about a 



