...give them a special phone!
from Modern Mechanix (February, 1932)
Telephone companies are much concerned by eavesdropping on rural party lines because it interferes with transmission over the line. To take down the receiver increases the electrical resistance of the circuit.
It is proposed to stop fighting the apparently incurable tendency of rural subscribers to listen to other people’s business and to recognize it by installing special telephone instruments to which eavesdroppers can listen without increasing the electrical resistance of the circuit or interfering with its legitimate use. (more)
