MAGNETISM AND ELECTROMAGNETICS 



51 



Unit Current. If a conductor carrying one absolute unit of cur- 

 rent is placed in a magnetic field of unit strength at right angles 

 to the lines of force, each centimeter of its length will be acted 

 upon by a force of one dyne. 



The practical unit of current is one tenth of this absolute unit 

 and is called the ampere. 



In Fig. 41 (a) AB is a coil of wire carrying current placed in a 

 magnetic field. A is acted upon by a force /A tending to move it 

 down and B is acted upon by a force /# tending to move it up. 

 Since A is in the stronger field /^ is greater than /# and the coil 

 will move down to the position AI B\ where it is inclosing the 

 maximum flux. 



Fig. 41 (6) shows the direction of the forces acting on the coil in 

 another position. The coil is forced around until it reaches the 

 position AI BI. 



43. Transformation of Mechanical Energy to Electrical 

 Energy. In Fig. 42, if the conductor is moved through a distance 



FIG. 42. Transformation of mechanical to electrical energy. 



dx at right angles to the flux against the force /, the work done is 

 dw = fdx = ^>ll dx = SZ dx I, 



but %&l dx is the flux cut in moving through the distance dx and is 

 = d<f>, therefore 



dw = Id4, (81) 



and the work done in moving a current across a magnetic field is 

 equal to the product of the current and the flux cut. 



