THE TRANSFORMER,. 145 



rather higher values of the saturation. At the beginning 

 of the magnetisation curve, therefore, the increase of current 

 is rapid compared with the magnetic field produced. 



Curve II. is the true magnetisation curve for the trans- 

 former, and is similar in form to the magnetisation curve 

 for iron, as will be at once recognised on viewing the curve 

 with the figure turned so that the ampere scale becomes 

 horizontal. 



Transformers are always designed to work at low-core 

 saturation (say 4,000 5,000 lines per cm. 2 ), compared with 

 the density employed in direct-current magnetic circuits. 



This is necessary on account of the high iron losses which 

 would otherwise result from the rapid alternation of the field. 

 The " knee " of the magnetisation curve, which is so im- 

 portant in direct-current work, is never reached in the 

 magnetisation curve of a transformer. 



The experiment just given has a further important 

 application, owing to the fact that the core losses of a trans- 

 former are very nearly the same at all loads as they are at 

 no-load. The experiment is therefore a determination of 

 the core losses of the transformer when the induction in the 

 magnetic circuit is such as to produce the measured voltage 

 at the terminals of the winding connected to the voltmeter 

 in the test. If the winding supplied with current is the 

 low-tension winding, the wattmeter will indicate the core 

 losses under working conditions, if the voltage applied to this 

 coil is the working voltage of that winding. 



From the results of this experiment it is consequently 

 possible to calculate the efficiency of the transformer at all 

 loads if the resistance of the windings is known, since the 

 losses which occur in working consist of the iron losses and 

 the O'R losses in the windings. 



A simple method of determining experimentally the 

 copper losses (i.e., the losses in the windings) is given later 

 (Experiment XXVIII.), and may be considered in some 

 respects as forming the complement of the 'experiment just- 

 described, although it is less interesting in other respects. 



No-load Losses Due to Defective Insulation. If the no-load losses 

 of a transformer are found to be too high, and the core is of 

 normal weight and has not been damaged, and if the ratio of 

 transformation is also normal, the excessive loss may probably 

 be traced to defective insulation, this is especially likely 



