28 Proceedings of tJie Royal Society of Victoria. 



The approximate value of r required may be found by a rough 

 preliniiuary calculation, or from a formula such as that given in 

 § 55, when r for some other transformer of the same type is 

 known, and 0^, the fall load value of <j), taken as given by the 

 equation (see ^ 17), 



where z is the chosen ratio of copper to iron losses at full load. 



In Section III. of this paper will be shown how to determine 

 the leakage coefficients when the form of the magnetic circuit, 

 method of winding, and space factors, are known. As the.se 

 details will be decided on in the first place, a fairly accurate 

 preliminary value of x^ can be obtained, and hence the value of 

 Cos^' by means of either of the equations given above. 



Thus we can obtain from equation I. above the product of the 

 section of the iron circuit by the total section of the secondary 

 copper circuit. 



Obviously, as a first approximation we might in equation I. 

 consider P'2 = P2 and Cos^' = Cos0, which would amount to 

 neglecting secondary leakage and secondary copper loss. 



A second relation between the two variables is obtained by 

 expressing the condition that, at full load, the ratio of the copper 

 to the iron losses is to have a definite chosen value z. 



If s be chosen as unity for a transformer that is to operate a 

 non-inductive load, or as 



(using the approximate value of r) for one that is to operate an 

 inductive load whose power factor is Cos^, then in either case 

 full load would correspond with maximum efficiency (see § 16). 



As the efficiency of a transformer keeps very near its maxi- 

 mum value for a wide range on either side of the maximum 

 position, it is not a matter of great importance to arrange that 

 maximum efficiency exactly corresponds to full load. 



As copper costs more than iron it may be more economical to 

 use a relatively smaller quantity of copper, and put up with a 

 larger copper loss than when s=l. 



