Patent GB-188701-A (GB)
Title: Improvements in or relating to electric telegraphy
IPC: H04L25/02
Filed: 1921-06-14
Abstract: 188,701. Pernot, F. E. June 14, 1921. Automatic telegraphs; perforated-strip transmitters; alternating-current systems; systems using code elements distinguished by duration and by polarity of current; systems using code elements distinguished from spaces by polarity of current; systems using code elements represented by half-waves of alternating current.-In telegraph systems in which the sources of electromotive force used are introduced and removed in such a way that the applied electromotive force, plotted against time, gives a continuous curve of the kind shown in Fig. 5, means are provided whereby the signalling circuit is never broken, and preferably also whereby the circuit impedance is kept approximately constant, with the object of avoiding discontinuity in the effective electromotive force in the line as distinguished from the applied electromotive force. The requisite unidirectional electromotive force varying between zero and a maximum may be obtained by a known arrangement of an alternating and a direct current source in series. Detailed description is given of the use of the invention in cable-code, single-current Morse code, and double-current Morse code signalling. In a simple arrangement for cable-code signalling, a reversing switch is used for making and breaking connection between the source of electromotive force and line when the electromotive force has zero value and zero rate of change. A separate switch in series with an impedance equal in value to that of the current source is used to earth the cable immediately before breaking a reversing switch connection, so as to avoid any break in the signalling circuit. A preferred arrangement, Fig. 7, which allows more rapid signalling, makes alternate use of two switches 29, 30 alternately connected to line by slip rings 23, 24. The source of electromotive force 15 comprises sources 15<1>, 15<11> of direct and alternating electromotive force respectively. The slip rings are geared to the alternator so as to make one revolution every two cycles, each switch being connected for the period of one cycle. A slight overlap of the conducting segments of rings 23, 24 provides for the connection of cable 28 to earth 27 through one key before connection with the other key is broken. A modified arrangement of switch with impedances is described for use in place of switches 29, 30, if desired to keep the circuit impedance constant. The above arrangement is preferably operated by means as shown in Fig. 8, comprising electromagnets 31<1> - - 34<1> which respectively operate the keys 31 - - 34 of the switches 29, 30, Fig. 7. Cams 35, 38, 41, each making one revolution every two cycles, control the contacts 36, 37, 39, 40, 42, 43. A contact arm 44, controlled by perforated tape in a known way, makes contact with contacts 45, 46 according to whether it is desired to send dot or dash impulses. Assuming keys 31, 33 are used for dots, and keys 32, 34 for dashes, the operation of sending the impulses indicated in Fig. 5 is as follows. Arm 44 contacts with contact 46 to send a dot and, contact 40 being closed, magnet 31<1> is energized, closing a hold-on circuit and maintaining key 31 depressed. The impulse is then transmitted by the slip rings, the arrangement being such that the line connection is made and broken when the electromotive force has zero value. Momentary opening of contact 43 de-energizes magnet 31<1> and releases key 31. In the meantime, arm 44 has again touched contact 46 to send the second dot and, contact 39 being closed, magnet 33<1> is energized to hold key 33 depressed, the required impulse being sent when key 30 is connected to line by rings 23, 24. Momentary opening of contact 42 releases key 33. In a similar way, contact of arm 44 with contact 45 energizes magnet 32<1> and depresses key 32 to send the dash impulse when the line connection of switch 29 is made by the slip ring, and contact between 44, 46 again energizes magnet 33<1> to send the final dot impulse. For single current Morse code signalling, in which the electromotive force variations corresponding with those of Fig. 5 would be as shown in Fig. 11, two independent sources of electromotive force are used, of the same frequency but opposed in phase. One source is used to send the dot impulses, the second being introduced to fill the hollow between two dot impulses, giving the flat-topped curve shown in Fig. 11, when it is desired to transmit a dash. A wiring diagram is given in the Specification, and an arrangement for controlling the switch operation is described in which a rotary member has an annular groove the opposite walls of which have cam grooves giving a switch member axial movement, the engagement of this member with one or the other of the opposed cam grooves being controlled. by an electromagnet. An adaptation of this arrangement to sending cable code messages is described in which the two sources of electromotive force are in phase but in opposite sense, one being used for dot impulses and the other for dash impulses. A further arrangement is described employing a four-segment commutator, rotation of which is controlled by an escapement arrangement controlled by an electromagnet. By inserting a source of steady electromotive force in the cable to shift the datum line, " double-current " Morse impulses may be sent having the character shown in Fig. 15, dots and dashes comprising impulses in one direction, and the intervals between them impulses in the other direction. An arrangement specially for this type of signalling is described using a direct current source, the connection of which to line may be reversed or broken, and an alternating-current source which can be connected to line in place of the direct source, the changes taking place at the required point of the cycle. Normally, the steady electromotive force is supplied to line in the negative sense. To send a dot, or the initial and final portions of a dash, the alternator is used, the direct source being disconnected. The flat top portion of the dash is provided by the direct source alone, connected to line in the positive sense.