TELEGRAPHY — HILLIS 195 



Any disturbance to the earth potential will seriously affect operation 

 over this type of circuit. On the morning of March 23-24, 1940, 

 approximately 800 volts difference in earth potential was observed 

 between New York City and Binghaniton, N. Y. This means that 

 with one of the wires grounded at Binghaniton, the New York test- 

 board attendant would see on his voltmeter connected to the New 

 York end of the wire, not the Binghamton ground but anywhere 

 from to 800 volts of battery and varying from positive to negative. 

 At times the maximum potential would hold steady for 30 seconds 

 or more, decrease in intensity, then suddenly reverse potential and 

 increase in intensity until approximately 800 volts of the opposite 

 potential was reached. Under these conditions, telegraph signals of 

 160- or 240-volt potential were entirely obliterated. 



Trouble is also experienced during very wet weather when a thin 

 film of water on the glass insulators acts as a high resistance conductor 

 and allows a small portion of tlie transmitted battery to leak off to 

 ground at each telegraph pole and return to the transmitting station. 

 Although this is a very small amount at each pole, when you have 

 a 200-mile circuit with 40 poles per mile, 8,000 such leaks do not allow 

 much of the transmitted signal to get through to the distant end. 



When a second wire is substituted for the ground return, many of 

 the above troubles are eliminated. This type of operation requires 

 twice the wire facilities for the same number of circuits. One type 

 of metallic system used by the telegraph company provides three 

 excellent circuits from four wires. 



In order to provide a more stable means of transmitting intelligence 

 between terminals which would not be affected by the inherent hazards 

 of grounded operation and provide a number of circuits over a pair of 

 wires, the Western Union began experimenting with carrier operation 

 in 1927. 



The first 4-wire, amplitude-modulated carrier system was placed 

 in service between New York City and Buffalo. Wliile this system 

 was a great improvement over physically grounded operation, it needed 

 many improvements. 



The B-3 system was placed in operation a short time later between 

 New York and Chicago. This was a 4-wire, amplitude-modulated 

 system originally designed for 20 operating channels. Channel fre- 

 quencies ran from 450 cycles to 6,450 cycles with a 300-cycle channel 

 width. Carrier current for the individual channels was provided 

 by a Hammond generator, an adaption of the same machine that 

 supplies the basic tones for a Hammond electric organ. 



The addition of an automatic bias corrector to the channel terminals 

 to compensate for changes in the received level, made these circuits 

 far better than any ground return circuits for the operation of high- 



