DESIGN OF TRANSFORMERS. Ill 



Let the iron and copper losses be each 150 watts at full load. 

 Working at the same current density in both the primary and 

 secondary circuits, the consequent dissipation of energy in each 

 will be 75 watts. 



67. Copper Circuits. Let li be the length of the 

 primary circuit, o-i its cross-section, and r\ its resistance ; and let 

 ^2, ""2, ^2 have similar meanings for the secondary circuit. If p is 

 the specific resistance of copper, we have 



(i) 



The full-load primary current is 



W 



^=-E l 

 = 10 amperes (approximately) ... (2) 



and the secondary current is 



i% 100 amperes ...... (3) 



We have, therefore 



?i 2 ^i 4- 4 2 ^2 = 150 watts 



If the two circuits are wound with wire, so that the current density 

 in them is the same, we have 



i-fi\ = ir% = 75 watts ..... (4) 

 and 



: - = - = 160 amperes per square centimetre 



<TI <T2 



working at about 1000 amperes per square inch, therefore 



= 0*0625 square centimetre .... (5) 

 and 



= 0'625 square centimetre .... (6) 



