Telephony, September 11, 1920, page 22:
Power Company Experimenting with "Wired Wireless."
Successful telephone communication has been held over live high tension lines by the American Gas & Electric Co., which has been convinced thereby that the method employed will solve one of its most important problems, namely, insuring a reliable and less expensive mode of communication between its load dispatches and interconnected stations.
The test which proved the practicability of the method, was conducted during July between two of the company's stations, over a live 11,000-volt, 60-cycle transmission line, 12 miles long. Between the transmitting and receiving sets were the windings of the power transformers at both ends of the line and an underground cable, making the equivalent length of transmission about 21 miles.
The system employed works on the principle of the wired or directed wireless, and involves the use of apparatus to tune out the power-circuit frequency. The carrier current for the conversations had a frequency in excess of 5,000 cycles. The transmitting and receiving sets were connected with the 2,300-volt buses at each station, but they could have been attached to the 440-220-volt circuits with just as satisfactory results, according to engineers who have developed the system.
Among the chief advantages of the scheme are that the investment and maintenance expense connected with private telephone lines are eliminated; the necessity of paying high rental charges for leased telephone lines is avoided; no interference from power circuits or static is experienced; no disturbance to neighboring telephones is produced; the system can be used with any voltage transmission; the possibility of the power circuits breaking is remote, and even if they all fall to the ground, communication can still be maintained provided there is not a dead earth on the lines.
The investment for apparatus will be only about $500 a station, which is more economical than an ordinary wired telephone system, and especially so the farther apart the stations are. According to the operators who communicated over the system, the audibility is higher than with ordinary telephones.
The American Gas & Electric Co. is planning to apply the communication scheme to all of its properties in various parts of the United States for system load dispatching.