ELEMENTARY ELECTRICITY AND MAGNETISM. I? 



Direction of current. A magnet pole tends to move round 

 an electric wire as stated in Art. 9. The current is by common 

 consent considered to flow along the wire in the direction in which 

 a right-handed screw coaxial with the wire would travel if turned 

 in the direction in which a north pole tends to move round the 

 wire. 



The relative directions of a current in a wire, the magnetic 

 field 3{ in which the wire is placed, and of the side push F acting 

 on the wire may be determined as follows : The field due to the 

 current circles round the wire in a direction which is easily deter- 

 mined from the above convention as to the direction of current, 

 when the direction of the current is given. This field due to the 

 current works against or weakens the field 3f on one side of the 

 wire, and strengthens the field c^ on the other side of the wire, as 

 shown in Fig. 8. The wire is pushed away from the side upon 

 which the composite field is intense. 



12. Intensity of the magnetic field at the center of a circular coil of 

 wire. Fig. 9 shows a circular coil of wire of radius r through 

 which a current of strength / 

 is flowing. It is required to de- 

 termine the intensity, at the 

 center of the coil, of the mag- 

 netic field due to the current in 

 the coil. Imagine a test pole of 

 strength m placed at the center 

 of the coil. This test pole pro- 

 duces a magnetic field which is 

 everywhere at right angles to the 



wire of the coil, and of which the intensity at the wire is mfr 2 , 

 by equation (3). The action of this field on the wire is to push 

 the wire sidewise (towards the reader in Fig. 9), and, by equation 

 (6), the force with which the wire is pushed is / X length of wire 



this determination the International Electrical Congress of 1893 defined the ampere 

 as that current which will deposit 0.001118 gram of silver per second from a solution 

 of pure silver nitrate. 

 2 



