256 



ADVANCED ELECTRICITY AND MAGNETISM. 



centimeter of length, and let / be the current in the wire in 

 abamperes, then RI is the intensity of the surrounding electric 

 field in abvolts per centimeter. The intensity of the magnetic 

 field at a distance of r centimeters from the axis of the wire is 

 2l/r gausses.* The intensity of the energy stream (ergs of 



i ; ii 



lines of force of electric field 



side view 



lines of force of magnetic field 

 end view 



Fig. 198. 



energy per second per square centimeter) at points distant r 

 centimeters from the axis of the wire is proportional to the 

 product of the electric and the magnetic field intensities, and it 

 may therefore be written k X RI X 2l/r, where k is an un- 

 known proportionality factor. Multiplying this expression for 

 the intensity of the energy stream by the area a of a cylindrical 

 surface I centimeters in length and r centimeters in radius 

 (cylindrical surface coaxial with wire), we have the total ergs per 

 second streaming into / centimeters of the wire, and this must 

 be equal to / X RI 2 . Therefore we have 



2wrl 



~ 



whence 



* See Art. 19. 



