SOME ASPECTS OF NOISE INDUCTION 



475 



which longitudinal currents will flow. The net v'oltage to ground 

 under these conditions is equal to the total of the longitudinal currents 

 in the two directions times the impedances to ground looking in the 

 two directions considered in parallel and, since these impedances are 

 usually much smaller than the impedance through which the current 

 reaches the telephone line (capacitance Ctp), this voltage is usually 

 much smaller than the induced voltage (see equation above). Since 

 the impedance of Ctp controls the total longitudinal current, this cur- 

 rent will be practically independent of the telephone circuit impedances 

 to ground and will be proportional to exposure length. It will also be 

 proportional to the frequency of the harmonics in the inducing voltage 

 (since the impedance of a capacitance is inversely proportional to 

 frequency). Hence, for given telephone circuit impedance conditions 

 (outside the exposure) the voltage to ground will be proportional 

 to exposure length and to the frequency of the inducing harmonics in a 

 uniform (electrically short) exposure. 



Fig. 3 — Fundamental of magnetic induction. 



Considering magnetic induction, the current in the power wire sets 

 up a magnetic field which alternates at the frequency of the current. 

 If a telephone wire is located in this field, a voltage is induced along 

 it which is proportional to the rate of change of the magnetic flux just 

 as a winding in a transformer has a voltage induced along it. This phe- 

 nomenon is illustrated in Fig. 3. The voltage between the telephone 

 circuit and ground varies from point to point along the circuit and 

 depends on the distribution of the impedances to ground as well as on 

 the distribution of the induced voltage. Also since the voltage acts 

 along the circuit and the part induced in each short length adds directly 



