PRINCIPLES OF ALTERNATING CURRENT 211 



the iron core. In a small coil with an iron core used in the 

 laboratory for testing purposes, the conductor resistance 

 is 0.70 ohm and the effective resistance varies from 4 ohms 

 to 30 ohms, depending upon the current and frequency. 

 In the case of an aerial conductor used as an antennae for 

 sending messages by wireless telegraphy, the conductor 

 resistance may be quite small, but the effective resistance 

 is much greater owing to the energy that leaves the circuit 

 in the form of electric waves. 



Skin Effect. When the alternating current is of high 

 frequency another effect comes into play to increase the 

 resistance of a conductor; it is called the skin effect. It 

 is found that, when the conductor is of appreciable diameter 

 (say, 0.25 inch) and the frequency is high (say several 

 thousand), the current does not utilize all of the cross- 

 section of the conductor but flows almost entirely in a thin 

 layer of the conductor near the surface. In fact, for very 

 high frequency currents a solid rod is not appreciably a 

 better conductor than a thin tube of the same material 

 of the same diameter. Of course, this effect increases the 

 effective resistance of a conductor. 



51. Method Used for Solving A-C. Circuits. In a 

 circuit containing only resistance (no inductance or capacity) 

 the impressed e.m.f. is all used up in overcoming the rssist- 

 ance reaction. This is expressed by Ohm's law, written 

 in the form 



E=IR, 



where E is the impressed e.m.f. and IR is the resistance 

 reaction (explained in next paragraph). This idea of 

 equating the impressed force to the reactions existing in 

 the circuit will be very useful in discussing inductance and 

 capacity. We shall attempt to show how all problems in 

 a-c. circuits may easily be solved by use of the general 

 equation, 



Impressed force = sum of all reactions in the circuit. (46) 



