METHODS OF TESTING TRANSFORMERS. 119 



of insulation to that of iron becomes greater and greater, and for 

 a given volume, the magnetic flux through the core less and less. 



On the other hand, there is an upper limit to the thickness of 

 the stampings, for when iron is subjected to an alternating magne- 

 tizing force, the induced currents only penetrate to a depth 

 depending upon the permeability, the frequency, and the specific 

 resistance. This is known as the "skin" effect, and it can be 

 shown mathematically 1 that we may take as a measure of the 

 thickness of the " skin," the expression 



where a is the specific resistance of the iron, /u its permeability, 

 and^> is 2/r times the frequency. 



If n = 1000, and the frequency is 100 periods per second, the 

 skin for soft iron is about half a millimetre thick, and for a 

 frequency of 50 periods the skin is about 07 millimetre, or 

 about 27 '3 mils, thick. Thus, unless the laminations are less than 

 27'3 mils., they produce no beneficial effect. 



METHODS OF TESTING TRANSFORMERS. 



72. There are several methods of testing transformers for 

 efficiency, each of which has its advantages in special cases. In 

 each method the watts given to the primary circuit and taken 

 from the secondary circuit at various loads are measured or 

 calculated, and the efficiency determined as the ratio of the out- 

 put to the input. 



73. First Method (Wattmeter) Suppose that we 

 have at our disposal the transformer whose efficiency is to be 

 determined, an alternator, or some source, capable of supplying 

 power to the transformer from no-load to full-load, an alternating- 

 current ammeter and voltmeter, a wattmeter, and a set of non- 

 inductive resistances, with switches and connecting cable. 



The terminals of the alternator should be connected through 

 a resistance to the primary terminals of the transformer. The 

 object of this resistance is merely to prevent an abnormal rush of 



1 See "Recent Researches in Electricity and Magnetism," J. J. Thomson, 

 Clarendon Press. First edition, 1893, p. 281. 



