120 THE DESIGN OF STATIC TRANSFORMERS 



convection of heat in a horizontal direction. It takes prac- 

 tically about five hours before the volume of oil at G is heated 

 up. Immediately after this stage is reached, heated oil begins 

 to fill up the space H, which process takes another three 

 hours, and during this time the rate of rise of temperature 

 proceeds more rapidly, owing to the narrower space. As I have 

 already pointed out, the question of the rate of temperature 

 rise has nothing to do with the size of the apparatus it is 

 merely a question of the ratio of two 

 effects, viz., the ratio of the cooling- 

 surface to the heat capacity of the 

 whole volume. The case of the oil- 

 transformer experiment is somewhat 

 complicated by the motion of the 

 oil." 



Prior to Prof. Epstein's explanation 

 of the curves of Fig. 76 (which 

 occurred in his reply to the discus- 

 sion on his paper), Mr. J. S. Peck had 

 offered the following comments : 



" I have made a large number of 

 tests of oil transformers, but I have 

 never before seen an accurate curve 

 which had a hump in it at the end of 



FIG. 77. Diagram show- 

 ing positions of ther- 

 mometers for the heat- 

 ing curves shown in 

 Fig. 76. 



four hours. The temperature curve should be a perfectly 

 smooth one. It is well known that wide differences in tem- 

 perature are obtained at different oil levels, as shown in the 

 curve, but it seems remarkable to me that the oil at the bottom 

 of the case should remain cold for so long a time." 



The temperature rise of a transformer when arranged in a 

 case containing air will be very much greater than the tem- 

 perature rise of the same transformer when contained in the 

 same case filled with oil. Instances of this for two trans- 

 formers of 5 kva and 50 kva rated capacit}^ respectively are 

 given in the curves in Figs. 78 and 79. 



