LOW-FREQUENCY INDUCTION 



597 



line this shielding may, as pointed out previously, be so large that 

 the voltage across the open is reduced to a fraction of the induced 

 voltage. 



Using the high voltage equipment which was used in connection 

 with the demonstration of protector operation, "acoustic shock" 

 can be demonstrated. Although strictly the term "acoustic shock" 

 should be used only with reference to the effect on a person subjected 

 to an abnormally loud sound, the term has also come to be used to 

 designate a noise (usually transient) in a telephone receiver, the in- 

 tensity of which is considerably higher than that of speech. It is 

 produced by an excessive voltage across the terminals of the re- 



NO.l 

 NO. 2 

 NO. 3 



I I 



3 ELEMENT 

 OSCILLOGRAPH 



Fig. 16 — Oscillograms showing voltages across protector blocks and resulting voltage 

 across circuit, and schematic of test circuit. 



The disturbances of this nature which are of primary interest in 

 inductive coordination work are those which are liable to be ex- 

 perienced when a voltage high enough to cause the breakdown of 

 protectors appears on a telephone circuit. This may be the result of 

 low-frequency induction or may be produced by other causes, such as 

 lightning or contacts between power and telephone circuits. Although 

 induced voltages usually appear in equal magnitudes on the two sides 

 of a circuit, the protector gaps on the two sides of the circuit discharge 

 in an unsymmetrical manner with the result that a voltage higher than 

 normal appears across the circuit. When this occurs a loud noise or 

 rattle is produced in the receiver of a telephone set bridged across the 

 circuit. 



Figure 16 shows oscillograph traces of voltages measured across 



