H TRANSFORMERS. [Exp. 



including the cross section of the magnetic circuit and size of 

 wire, but do not remove parts, destroy insulation or damage the 

 transformer in any way in seeking this information. Data fur- 

 nished by the maker can be used for this purpose. 



23. Compute the current density in amperes per square inch 

 and in circular mils per ampere, for the primary and the sec- 

 ondary windings. Current densities from 1,000 to 2,000 circular 

 mils per ampere are common, but less copper was often allowed 

 in early transformers. 



24. Compute the maximum value of the total flux in C.G.S. 

 lines or maxwells (see 9a, Exp. i-B) ; thus 



__ A v r> EX io 8 



A X >max. -= - 



where E is the voltage and 5 the number of turns for any coil, 

 and n is frequency. The quantity E -f- S is the volts per turn. 

 For proof of formulae, see Appendix II. 



Compute the maximum value of the flux density in gausses 

 (flux per sq. cm.) ; thus 



E X io 8 

 Flux density = ,. = = 



where A is the cross section* of the core in sq. cms. If A is in 

 square inches, 5 max . is the flux density in lines per square inch. 

 If A, in square inches, is multiplied by 6.45, the formula gives 

 m ax. in gausses for, unfortunately, this mixture of C.G.S. and 

 English units is in common use. 



25. The computations for B should be made for standard 

 frequency (60 cycles), and two other frequencies (30 and 120) 

 with the same value of E. If values of A and 5 are not obtain- 

 able, assumed values may be assigned for practice computations. 

 If the cross section of the core is not uniform, B will have dif- 



* (243). The net cross section is, say, 15 per cent, less than the gross 

 cross section on account of lamination. 



