THE TRANSFORMER. 139 



Or lines due to one coil = Z l = k Nfli, k being an 

 unknown constant. 



Also the current is equal to the voltage of the battery 



El 



divided by the resistance of the branch circuit or C l = -g-, 



ti\ 

 E being voltage of battery. 



Similarly, the lines due to the current in the secondary coil 



When the galvanometer is unaffected by the starting or 

 stopping of the current, the fields due to the two windings 

 exactly neutralise each other, i.e., Z l = Z 2 



The method of measurement just given is to be considered 

 as a convenient approximate test for use on occasions when 

 an alternating supply is not readily available rather than a 

 reliable method of determination. 



No-load Current of a Transformer. In order to produce 

 the magnetic field in the core of a transformer, a certain 

 amount of magnetising current is necessary. The amount of 

 this current is determined in the same way as the magnetis- 

 ing current of a direct-current electromagnet. At any 

 instant the value of the current is given by the formula 

 generally used for direct-current calculations : 



Ct= x -313 

 /* 



or Ct= x 2-02 



/* 



or, Ct = - B x -8 



/* 



where C = current in amperes. 



t = number of windings. 



I length of path in inches. 

 A. = length of path in cms. 

 B" = induction in lines per square inch. 

 B = induction in lines per square cm. 

 /* permeability of core. 



