MULTIPLEX TELEPHONY AND TELEGRAPHY — SQUIEE. 137 



is 5.4 ohms at 100,000 cycles. This would require a capacity of 

 about 0.3 microfarad for resonance at this frequency, but in the 

 experiments conducted at 100,000 cycles it was found necessary to 

 decrease this amount on account of the fixed auxiliary inductance of 

 the leads. 



CONSTANTS OF THE TELEPHONE LINE. 



The telephone line used in these experiments extends from the 

 Signal Corps laboratory at 1710 Pennsylvania Avenue to the Signal 

 Corps research laboratory at the Bureau of Standards. 



This line is made up of the regular standard commercial equipment 

 and consists of paper-insulated, twisted pairs in lead-covered cable, 

 placed in conduit in the usual manner employed for city installation. 

 For the sake of convenience, one of the pair is designated as No. 1 

 wire and the other as No. 2 wire. 



The air-line distance between the two laboratories is a little over 

 3 miles (4.8 kilometers), but the telephone line, by passing through 

 three exchanges, covers about 7 miles (11.27 kilometers). The course 

 of the line, with the size and type of conductor, is as follows : 



Laboratory to main exchange, underground cable, No. 22 B. & S. 

 Main exchange to west exchange, underground cable, No. 19 B. & S. 

 West exchange to Cleveland exchange, underground cable, No. 19 B. & S. 

 Cleveland exchange to Bureau of Standards, underground cable, No. 19 B. & S. 

 All underground cable except from Bureau of Standards to Wisconsin Avenue and 

 Pierce Mill Road, about 3,400 feet, which is aerial cable. 



This line is equipped with protective heat coils of a standard type, 

 one in each wire of the metallic circuit, at the Cleveland exchange and 

 the main exchange, but none at the west exchange. The constants of 

 each of these coils are as follows: 



Direct current resistance of 65° F ohms. . 3. 8 



Size of wire, No. 30 B. & S. 



Length of wire cm. . 40 



Number of turns in each coil, about 38 



Measured inductance at 70,000 cycles cm. . 4, 400 



Or 4.4 XIO -6 henry. 



The above constants were measured from a sample of one of these 

 coils selected at random. 



Resistance of metallic circuit ohms. . 776 



Capacity measured (one minute electrification) between No. 1 and No. 2 wires 



microfarad. . 0. 69 



Insulation resistance : 



Between No. 1 wire and earth megohms. . 0. 9 



Between No. 2 wire and earth do 1.3 



Between No. 1 and No. 2 wires in parallel and earth do 0. 8 



Between No. 1 and No. 2 wires do 2. 1 



The line included the usual house-wiring at each station, which was 

 undisturbed in taking the measurements. 



