432 Mr. 0. Heaviside on Telegraphic 



manent current which would flow were the end of the line put 

 to earth. 



The effect of a condenser on the signals varies according to the 

 description of instruments used for observing the signals, on the 

 system of currents forming the signals, and on the value of the 

 unit of time a for the particular line under consideration. Thus 

 on an overland line worked on the Morse system with single 

 currents, where the signalling consists in alternately connecting 

 the sending-end of the line with one pole of a battery and to 

 earth, it would be impossible to make dashes, because on land 

 lines the length of a contact is always so great compared with 

 the unit a, even in rapid signalling, that the current at the re- 

 ceiving-end would have come and gone long before the contact at 

 the sending-endwas finished. On cablesof any considerable length 

 it would be different. There would then be only a shortening of 

 the dashes, its extent depending on the length of the contact ; 

 and it would consequently lead to an increase in the speed of 

 working. There would similarly be very little direct advantage, 

 save immunity from earth -currents, in using on land lines the 

 condenser with polarized relays and reversing-keys, although 

 there would be no shortening of the marks, as the armature of a 

 polarized instrument will, if properly adjusted, remain in the 

 position in which it is placed by a transient current. On cables 

 worked with the same instruments, a considerable increase in 

 the speed of working results. 



But the condenser is peculiarly applicable to those systems of 

 signalling where the currents sent are of equal duration and 

 alternately positive and negative ; for example, Sir C. Wheat- 

 stone's automatic system. A succession of reversals, each con- 

 tact of the length 4«, produces through the receiving instrument 

 reversals alternately 50 per cent, plus and 50 per cent, minus, 

 being a whole range of 100 per cent. Without the condenser 

 the whole amplitude of variation of the current is only 24*42 

 per cent. On a certain circuit worked by the automatic system, 

 on which the value of a was about 0*0175 second, the speed of 

 working with condenser was 75 per cent, greater than without. 



With Sir W. Thomson's mirror and recording instruments 

 there does not at first sight appear any reason why the speed of 

 working should be much raised, as they indicate in the one case 

 and record in the other every change in the strength of the cur- 

 rent. Yet the condenser is of great advantage here, as it keeps 

 the spot of light and siphon within very narrow ranges, never 

 departing much from the zero line, and naturally the signals are 

 much more distinct. 



It will be found on examination that -j-, when the end of the 



