A novel form of double transmission was invented by the writer soon after the completion of the harmonic system, and was an outgrowth of it. It is still in use on some of the railroad-lines. An ordinary railroad telegraph-line has an instrument in circuit in every office along the road, chiefly for purposes of train-dispatching. As we have heretofore explained, whenever any one office is sending, the dispatch is heard in all of the offices. The "Way duplex" system permits of the use of the line for through business simultaneously with the operation of the local offices. That is to say, any station along the line may be telegraphing with any other station by the ordinary Morse method, and at the same time messages may be passing back and forth between the two end offices. This is accomplished by the following method: At each end of the line there is a tuned reed, such as we have described in our last chapter, that is kept constantly in vibration by a local battery during working hours. This vibrator is so arranged in relation to the battery that whenever the key belonging to it is depressed the current all through the line is rendered vibratory. There is also in circuit at each end of the line a harmonic relay, that is tuned in accord with the vibrating reed of the sender. If either key belonging to this part of the system is opened, as in the act of sending a message, these harmonic relays, being tuned in sympathy with the sending-vibrator, will respond, thus sending Morse characters made up of a tone broken into dots and dashes. This tone can be read directly from the relay, or, as is usually the case, it causes the sounder to operate in the common way. You will at once inquire why the ordinary Morse instruments in the local offices are not affected by these vibratory signals, and also why the harmonic instruments at the end office are not affected by the working of the local offices. The local office does not open the circuit entirely, but simply cuts out a resistance by the operation of the special harmonic key. When a resistance is thrown into an electric circuit it weakens the current in proportion to the amount of resistance interposed. You will see that there is some current still left in the line when the key is open, but the spring of the relay at the local office is so adjusted as to pull the armatures away from the magnets whenever the current is weakened by throwing in the resistance, so that by this means an ordinary Morse telegraphic relay may be worked without ever entirely opening the circuit. In the Way duplex system there is a resistance at each station that is cut in and out by the operation of its key, which causes all the instruments in the line to work simultaneously except the two harmonic relays located one at each end of the line. These will not respond to anything but the vibratory signal.