THE TRANSFORMER 



299 



rent. A reading of the primary wattmeter then gives the 

 full-load copper loss in both coils. (There is a very small 

 core loss included in this reading but it is ordinarily neg- 

 lected.) The copper loss for other loads may be deter- 

 mined by proportionality, if desired, without actually 

 measuring them. For instance, the half-load copper loss 

 will be one-quarter of the full-load loss; the quarter-load 

 loss will be one-sixteenth the full-load loss, etc. 



The reading from these two tests may also be used to 

 predetermine the power factor and the regulation of the 

 transformer; the student is referred to more advanced 

 text-books for the method. 



Wattmeter 



Wattmeter 



Ammeter 



FIG. 188. Connection of Meters 

 for Iron Loss Determination. 



FIG. 189. Connection of Meters 

 for Copper Loss Determination. 



75. The Autotransformer. This name is applied to an 

 ordinary transformer, the coils of which are connected 

 together electrically. Generally, the two coils of a trans- 

 former are connected together only by the magnetic field, but 

 in the autotransformer they are connected together not only 

 magnetically but also electrically. 



Example of an Autotransformer. Suppose an ordinary 

 1100-volt/llO-volt transformer, the coils of which are 

 electrically connected together, as shown in Fig. 190. Now, 

 instead of taking the secondary line from c-d, as would 

 ordinarily be done, let the secondary load be taken from 

 the points d-e. The voltage of this secondary circuit will 

 be that of the primary, increased or decreased by the voltage 

 generated in the coil c-d. If the coil c-d acts in* the circuit 



