216 Mr. 0. Heaviside on the Speed of Signalling 



to twice, four, or six times the time of a current*. The arma- 

 ture of the receiver is adjusted neutral, so as to remain on the 

 side any current sends it to, until an opposite current reverses 

 its position. Lines of two lengths are thus made: — a "dot" 

 by first a + current immediately followed by a — current 

 to terminate it, thus + — ; and a " dash " of three times 

 the length by first a + current, then an interval of insulation 

 for twice as long, and lastly a — current to terminate it, thus 

 + 00 — . At a speed much below the limiting speed the sent 

 signals are reproduced at the receiving-end without sensible 

 alteration ; but as the speed of working is increased and the 

 currents have not time to reach their full strength, irregulari- 

 ties show themselves, which increase rapidly as the length of 

 each contact is reduced, until at length a limiting speed is 

 reached at which some of the signals miss fire altogether. 

 Consider the succession of signals 



a b c d e f g h i j 



+ - + - + - + -0000 + - + 00 - + -0000 



(illustrating a typical failure), consisting of a series of dots, 

 followed, after an interval of insulation, by a dot, a dash, and 

 a dot. If the receiver is adjusted so as to record the dots a, b, 

 c, d perfectly, the signals # and j will fail, g will fail because 

 the — current e has time to die away during the interval of 

 no sent current 0000, thus making the succeeding + current 

 f too strong ; and j will fail because the + current h has 

 time to die away during the interval of no sent current 00, 

 thus making the — current i too strong. In the first case the 

 dot is continued on to the dash, in the second the dot is lost. 

 Thus, although generally, to get the greatest possible working- 

 speedy the retardation should be as small as possible, yet in this 

 system of contacts of equal duration to make lines of unequal 

 length, it is important that some of the currents, viz. those com- 

 mencing dashes or spaces, should not die away too quickly. 

 They are prevented from doing so, in a great measure, by the 

 insulation of the line at the sending-end during the intervals of 

 no sent current, which, by closing up the path at one end for 

 the charge to escape, prolongs the current at the other. (The 

 compensation currents, sent by an improved form of trans- 

 mitter, have for their object to still further lengthen out the 

 currents.) Now it will be seen from the figures that when A 

 insulates the line at h, fig. 1, the charge of the cable discharges 



through the resistance ~ + b + g, and that when B insulates 



at k f , fig. 2, it discharges through the smaller resistance 



* Mr. Culley's ' Handbook ' contains a full description of the apparatus- 



