Edison, His Life and Inventions [313]
essential difference between the neutral relay, as used in the duplex and as used in the quadruplex, may be given. With the duplex, as we have seen, the current on the main line is changed in strength only when both keys at OPPOSITE stations are closed together, so that a current due to both batteries flows over the main line. When a single message is sent from one station to the other, or when both stations are sending messages that do not conflict, only one battery or the other is connected to the main line; but with the quadruplex, suppose one of the operators, in New York for instance, is sending reversals of current to Chicago; we can readily see how these changes in polarity will operate the polar relay at the distant station, but why will they not also operate the neutral relay at the distant station as well? This difficulty was solved by dividing the battery at each station into two unequal parts, the smaller battery being always in circuit with the pole-changer ready to have its polarity reversed on the main line to operate the distant polar relay, but the spring retracting the armature of the neutral relay is made so stiff as to resist these weak currents. If, however, the transmitter is operated at the same end, the entire battery is connected to the main line, and the strength of this current is sufficient to operate the neutral relay. Whether the part or all the battery is alternately connected to or disconnected from the main line by the transmitter, the current so varied in strength is subject to reversal of polarity by the pole-changer; but the variations in strength have no effect upon the distant polar relay, because that relay being responsive to changes in polarity of a weak current is obviously responsive to corresponding changes in polarity of a powerful current. With this distinction before him, the reader will have no difficulty in following the circuits of Fig. 8, bearing always in mind that by reason of the differential winding of the polar and neutral relays, neither of the relays at one station will respond to the home battery, and can only respond to the distant battery--the polar relay responding when the polarity of the current is reversed, whether the current be strong or weak, and the neutral relay responding when the line- current is increased, regardless of its polarity. It should be added that besides the system illustrated in Fig. 8, which is known as the differential principle, the quadruplex was also arranged to operate on the Wheatstone bridge principle; but it is not deemed necessary to enter into its details. The underlying phenomena were similar, the difference consisting largely in the arrangement of the circuits and apparatus.[24]
[24] Many of the illustrations in this article are reproduced from American Telegraphy and Encyclopedia of the Telegraph, by William Maver, Jr., by permission of Maver Publishing Company, New York.
Edison made another notable contribution to multiplex telegraphy some years later in the Phonoplex. The name suggests the use of the telephone, and such indeed is the case. The necessity for this invention arose out of the problem of increasing the capacity of telegraph lines employed in "through" and "way" service, such as upon railroads. In a railroad system there are usually two terminal stations and a number of way stations. There is naturally much intercommunication, which would be greatly curtailed by a system having the capacity of only a single message at a time. The duplexes above described could not be used on a railroad telegraph system, because of the necessity of electrically balancing the line, which, while entirely feasible on a through line, would not be practicable between a number of intercommunicating points. Edison's phonoplex normally doubled the capacity of telegraph lines, whether employed on way business or through traffic, but in actual practice made it possible to obtain more than double service. It has been in practical use for many years on some of the leading railroads of the United States.
The system is a combination
[24] Many of the illustrations in this article are reproduced from American Telegraphy and Encyclopedia of the Telegraph, by William Maver, Jr., by permission of Maver Publishing Company, New York.
Edison made another notable contribution to multiplex telegraphy some years later in the Phonoplex. The name suggests the use of the telephone, and such indeed is the case. The necessity for this invention arose out of the problem of increasing the capacity of telegraph lines employed in "through" and "way" service, such as upon railroads. In a railroad system there are usually two terminal stations and a number of way stations. There is naturally much intercommunication, which would be greatly curtailed by a system having the capacity of only a single message at a time. The duplexes above described could not be used on a railroad telegraph system, because of the necessity of electrically balancing the line, which, while entirely feasible on a through line, would not be practicable between a number of intercommunicating points. Edison's phonoplex normally doubled the capacity of telegraph lines, whether employed on way business or through traffic, but in actual practice made it possible to obtain more than double service. It has been in practical use for many years on some of the leading railroads of the United States.
The system is a combination