LOW-FREQUENCY INDUCTION 591 



is impressed by means of a power transformer in order to secure a 

 sufificiently high voltage. A general idea of the relative magnitudes 

 of the voltages at the two ends can be had by observing the brilliancy 

 of the voltage measuring lamps. By varying the slide wires which 

 control the terminal impedances the proportions of the total voltage 

 which appear at either end are changed and an idea of the changed 

 distribution can be obtained by observing the changing glow of the 

 measuring lamps. Finally the voltage at one end can be increased 

 enough to cause the protectors at that end to operate, whereupon 

 the measuring lamp goes out and the small protector lamp lights. 

 Immediately the other protectors operate, as evidenced by the voltage 

 lamp going out and the protector lamp lighting, and the line current 

 increases as evidenced by the brilliance of the line current indicating 

 lamp. 



An important factor in the further analysis is the characteristics of 

 the telephone protector. The arc takes place between two carbon 

 surfaces. The gap between these two surfaces has a very high break- 

 down speed and a very low impedance after it is broken down. 

 Another important characteristic from the standpoint of low-frequency 

 induction is its tendency to become permanently grounded if heavy 

 currents are discharged or if the discharge continues for some time. 

 Consequently, the amount of current in the longitudinal circuit in the 

 event of a breakdown and its duration are important factors in de- 

 termining the chance of permanently grounding the protectors and 

 causing the circuit to become inoperative until the blocks are changed. 

 Duration is, of course, ordinarily a function of the duration of fault 

 current on the power line as pointed out previously. 



The amount of current through operated protectors is determined by 

 the longitudinal voltage and the longitudinal impedance of the tele- 

 phone circuit. If, for the moment, it is considered that only one wire 

 is present, this current is simply the total longitudinal voltage divided 

 by the total series impedance of the wire plus any resistance in the pro- 

 tector grounds. This can be seen from Fig. 11. 



Ordinarily there are numerous circuits on an open-wire telephone 

 lead or in a telephone cable. If the protectors on a number of these 

 circuits break down, the current in each wire will be less than that 

 which would exist were only one wire present as in the above illustra- 

 tion. This is due to the mutual impedance between the different 

 telephone wires which causes the current in any one wire to reduce 

 the current in the remaining wires. The total current in all of the wires 

 of course increases as the number of wires on which protectors have 

 operated is increased but not in direct proportion. Figure 12 illus- 



