THE HEATING OF TRANSFORMERS 



133 



little pi in will be made by employing ribs of such dimensions as 

 inn-ease the surface by much more than two or three times, and 

 for such proportions the above data will hold fairly true. In the 

 early days of oil-immersed transformers it was sometimes the 

 practi.-e t" provide ribs both inside and outside of the case, but 

 til.- ribs inside are of relatively little use for the reason that the 

 heat is transferred from the oil to the case much more readily 

 than it can he transferred from the case to the surrounding air. 

 The heating data set forth in the preceding paragraphs is 

 presented in orderly form in Table 14. 



TAHLE 14. CONSTANTS KOI: mi: ESTIMATION 01 mi EsATHTG 

 OF Tl: LK8TOILMEB& 



Mi ui.'Ds OF MEASURING TK.MI -i I;AI TRES 



The 'guarantees given by manufacturers for the temperature 

 rise of tlieir transformers must be taken with caution unites 

 tin- metiiod of determining the tenipeniture rise is clearly 

 stated. That considerable precision of statement is nece^ary 

 wh.-n framing temprratun- .Lfuaruntees will appear from the 

 following i-onsiderations : 



In the first case there may often be a ditVen-nce -f tem- 

 ur- nf s..mr r to 1-2 or more, hetween measurements of 

 the vemttaneet of tin- windings and mca-urenn-nts by ther- 

 iiioin, t,-r> placed in the hottest accessible j the oil. 



This difference is, of course, readily leepunted for. Also 

 the ratio of the maximum tnnperature of the oil in 



