THE TRANSFORMER. 177 



The preceding table shows the method of entering up 

 results, and gives a few readings taken from an actual test 

 by way of illustration. The complete curves obtained in 

 the experiment are shown in Fig. 82, which shows the 

 following curves : (1) Joint efficiency of the two transformers ; 

 (2) efficiency of each calculated from (1); (3) curve of copper 

 losses ; (4) curve of iron losses. 



The transformers experimented upon were the two 1-kw. 



100 



- volt transformers for which a number of curves have 

 50 



already been given. 



Maker's Tests of Transformer. The object of the most 

 usual tests applied to transformers at the maker's works 

 where they are made in large numbers, is not so much the 

 determination in each case of the efficiency or quality of 

 design, as to ascertain if each individual machine corre- 

 sponds fairly to the standard transformer which was 

 thoroughly tested and experimented upon at the time of 

 adoption of that particular type. 



Thus the transformers to be tested are mostly such as have 

 a known ratio of transformation, efficiency, winding, &c., and 

 the test is only to ascertain that no individual transformer 

 is sufficiently unlike the rest through any accidental pecu- 

 liarity, or error, as to render its return to the shops necessary. 



It is to be expected that slight variations in the quality 

 of the sheet iron forming the core will render unavoidable 

 a certain want of uniformity in the no-load losses. A maximum 

 value for the loss must be fixed, and transformers showing a 

 greater loss must be rejected. Thus, for instance, if 2 

 per cent, at no-load is the normal value it may be decided to 

 reject those giving more than 2-4 per cent. loss. 



If a transformer with normal core is found to have 3 

 per cent, magnetising loss, it may be assumed that the 

 windings are too few and the induction too high. This 

 might be due either to too few windings having been wound 

 on, or to a short-circuit amongst windings, so that some are 

 cut out. Either of these faults would be detected on com- 

 paring the ratio of transformation with that of a normal 

 transformer. A measurement of the resistance of the 

 windings would probably point out the defect, but the 

 difference in resistance might possibly be masked by the 

 increase of the specific resistance of the wire, produced by the 



