OF ELECTEIC SIGNALS THEOUGH STJBMAEINE CABLES. 
993 
connexion with earth after the two signals is only of equal duration, it is clear that the 
received current will not fall so low after the dash as after the dot, and this is precisely 
the effect shown in the curves. The beginning and end of the dot are at different 
heights, and similarly the beginning and end of the dash are at different heights, making 
altogether a very irregular curve, especially at the higher speeds. 
This irregularity very seriously interfered with the distinctness of the received signals, 
although the change of current could be followed throughout by watching the spot. 
At a high speed the dot appeared as a mere pause followed by a fall, instead of a little 
rise followed by an equal fall ; and if the dots "and dashes had been irregularly combined, 
it would have been impossible to disentangle them, as received, by the eye. If a 
receiving instrument had been used like the common Morse receiver, simply marking 
the time during which the received current was above or below a given strength, the 
signals would have failed to give any intelligible record even at a very low speed : this is 
shown by the fact that it is impossible to draw a horizontal line intersecting both the 
curve of repeated dashes and that of repeated dots, even for the very lowest speeds 
recorded in each Table. 
Thus, long before the limit is reached at which signals cease to produce any change 
at the receiving end, the interference of one signal with another causes so great a con- 
fusion in the currents received as to put a fresh limit to the practicable speed of trans- 
mission. 
This confusion is still further increased by the effect of a pause in the signals between 
letters, words, or sentences. 
During all intervals the cable is left in connexion with the earth at A ; and if the 
pause lasts a little while, the current at X falls to nothing, or nearly nothing, when the 
effect of the first signals sent is to cause an unintelligible succession of sudden incre- 
ments in the received current, until, after a certain number of contacts, a permanent 
mean strength of current is attained, at which regular signals might become intelligible. 
The higher the speed the greater the number of contacts required for this purpose ; 
for instance when 1802 knots were in circuit, the following Table gives the number 
of dots required to bring the spot to the mean position in which it was maintained by 
regular signals. 
Beats of metronome. Dots required to raise current 
to 42 per cent, of maximum. 
92 6 
84 6 
72 4 
60 3 
50 
40 2 
The curve in fig. 3, Plate XLIX. roughly represents the variations of the received 
current when the operator begins to send the dots regularly through a cable which has 
