136 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



the primary coil by the reversals of magnetization of the core (resistance of primary 

 coil being neglected). Therefore, the electromotive force which acts upon the primary 

 coil is equal and opposite to the electromotive force which is induced in the primary 

 coil by the magnetic reversals of the core. The magnetic reversals of the core, how- 

 ever, induce a certain electromotive force e in each turn of wire surrounding the core. 

 Therefore the total electromotive force induced in the primary coil is Z'e and the 

 total electromotive force induced in the secondary coil is Z"e so that the ratio of the 

 two electromotive forces is equal to Z' ' \Z". 



(6) Current relations. The electromotive force which is induced in the primary 

 coil of a transformer balances the electromotive force which is applied to the primary 

 coil as explained above, and the range of reversals of magnetization of the core must 

 be such as to induce this reacting electromotive force in the primary coil. There- 

 fore the combined magnetizing action of primary and secondary coils is always such 

 as to magnetize the core to that degree which will make the reacting electromotive 

 force in the primary coil equal to the electromotive force of the alternator which is 

 forcing current through the primary coil. 



When the secondary coil is on open circuit, just enough current flows through the 

 primary coil to produce the degree of magnetization above specified. Let this value 

 of the primary current, which is called the magnetizing current of the transformer, 

 be represented by i. When a current l ff is taken from the secondary Coil a current 

 F in addition to the magnetizing current i flows through the primary coil. The 

 current i still suffices to magnetize the core, and the magnetizing action of I" is ex- 

 actly neutralized by the equal and opposite magnetizing action of I' . The magnetiz- 

 ing action of I" is measured by the product Z' f I" and the magnetizing of /' is 

 measured by the product Z' I', so that, ignoring algebraic signs, we have 



Z'l' = Z"l" 



74. Eddy currents. Lamination. When an iron rod is mag- 

 netized or demagnetized, the changing magnetic flux through the 

 central portions of the rod induces electromotive forces around 

 the outer portions of the rod, and these electromotive forces pro- 

 duce what are called eddy currents. Eddy currents are also 

 produced in a mass of metal which is near to a moving magnet 

 or which moves in the neighborhood of a stationary magnet. 



Lamination. Those parts of electrical machinery which are 

 subject to rapid and frequent changes of magnetization are always 

 built up of iron wire or of thin sheets of iron so as to leave the 

 iron continuous in the direction of the magnetization but discon- 

 tinuous in the direction in which the eddy currents tend to flow. 



