MAGNETISM AND ELECTROMAGNETICS 53 



The electric energy transformed in a circuit is the product of 

 the power and the time. The practical units of electric energy 

 are the watt second or joule, the watt hour and the kilowatt hour. 



1 watt hour = 3600 watt seconds = 2655.4 ft. Ibs. = 3.413 b.t.u. 

 = 0.001341 horse-power hours. 



The absolute unit of electric energy is the erg. 1 watt second 

 = 10 7 ergs. 



44. Intensity of Magnetic Fields Produced by Electric Cur- 

 rents. The following cases are of Flui 

 special importance: (A) At the centre 

 of a circular loop of wire carrying a 

 current I absolute units. (Fig. 43.) 



If ei-C is the field intensity at 0, the 

 centre of the loop, a magnetic pole of 

 strength m placed at this point will be 

 acted upon by a force 



/ = m&C dynes 



FIG. 43. Intensity of the mag- 

 in a direction perpendicular to the netic field at the centre of a 

 plane of the coil. circular coil of wire. 



The pole will produce at the wire a field of intensity 



3-C = 



and a flux density 



ra,. 

 c = lines per square centimeter, 



where r cm. is the radius of the loop. 



This field will act on the wire with a force 



/i = Z&ll dynes 



in a direction perpendicular to the plane of the coil, where 

 I = 2irr is the length of the wire in centimeters. 

 Substituting the values of %& and I gives 



. m , _ T 2 irl , 

 /i = -5 X 27rr7 = m dynes, 



but the forces / and f\ are equal and therefore 



27T/ 



= m > 



r 



