AUTOMATIC PRINTING EQUIPMENT 413 



the line winding was de-energized. The rate of vibration was adjusted 

 to be approximately the same as the frequency of the transmitted 

 signals and the amplitude of the vibrating current was adjusted to be 

 approximately equal to the received signalling current so that the 

 latter, if of one polarity, would neutralize the effect of the vibrating 

 impulse and prevent the movement of the relay armature and if of 

 the same polarity would aid the vibrating impulse. The effect of this 

 combined action of the vibrating and received signalling impulses is 

 to reproduce, in the local circuit, signals of approximately the same 

 shape and duration as the original transmitted signals. 



The frequency attenuation characteristic of a system comprising a 

 long telegraph cable and its signal shaping amplifier and networks 

 when the latter are adjusted for the maximum transmission speed is 

 such as to cause the impulses of unit length, which represent half 

 cycles of the fundamental signalling frequency, to be received in 

 considerably smaller amplitude than the impulses of two units (or 

 more) length which represent half cycles of one half (or less) the 

 fundamental signalling frequency.^ The highest signalling speed 

 obtainable on a given cable is therefore determined by the length of 

 the shortest impulses which must be received in sufificient amplitude to 

 exercise control over the receiving apparatus and at that speed the 

 two-unit and longer impulses will be received In much greater amplitude 

 than is necessary for operation of the receiving apparatus. Gulstad 

 pointed out ^ that as the received impulses of unit length always occur 

 in the proper direction to aid the vibrating impulses they may therefore 

 be greatly reduced in amplitude without impairing the accuracy of 

 reception. On account of this fact the speed of signalling may be 

 increased to a point where only the two-unit and longer Impulses are 

 received in sufficient amplitude to overcome the effect of the locally 

 generated vibrating Impulses and control the movement of the relay 

 armature. At this Increased speed the impulses of unit length will 

 be either greatly diminished In amplitude or entirely removed by the 

 attenuating effect of the cable and at such times the armature of the 

 vibrating relay will be operated by the locally produced Impulses. 



As the rate of vibration of the Gulstad relay was determined entirely 

 by the values of the resistances and capacities in the local vibrating 

 circuit, the vibrations of the relay armature did not exactly coincide 

 either In frequency or phase relation with the signals sent by the 

 distant transmitter so that complete restoration of the incoming signals 

 to their original form was impossible and full advantage of the speed 



^ The fundamental signalling frequency is defined as the fundamental frequency 

 of a train of alternate positive and negative impulses of unit length. 

 ' K. Gulstad, loc. cit. 



