994 ME. E. JENKIN’S EXPEEIMENTAL EESEAECHES ON THE TEANSMISSION 
been fully discharged. Similar but converse effects would be observed if the line were 
left long in contact with the battery before the signals began. 
This irregularity has hitherto put the practical limit to the rate of transmission 
through long submarine cables. When this rate is exceeded, a dot sent may at one 
time cause a great rise in the received current, at another time it will barely arrest a 
fall, and thus similar signals at one end are found to produce utterly dissimilar effects 
at the other. No change in the battery has the smallest effect on this interference, nor 
can any delicacy in the receiving instrument disentangle the confusion, which, it should 
be observed, is totally distinct from the retardation of the signals. 
It would matter little that signals should appear in America even a minute or so after 
they had been made in England, provided the same signal at the sending end always 
produced the same effect at the receiving end. The experiments show how far this is 
from being the case. 
The usual mode of avoiding this confusion is to signal at so slow a rate that every dot 
causes a very large percentage of variation in the received current, while the dash causes 
nearly the maximum current which would be received from a permanent contact with 
the battery. 
In air lines, or in short submarine cables, or in long submarine cables with large con- 
ducting wires thickly covered, this plan does not entail too slow a speed for practical 
work ; but to avoid confusion in this manner when working through 2200 knots of the 
Red Sea cable, it would be necessary to reduce the speed to less than 10 dots per 
minute, whereas 40 dots or more could be received but for the interference. 
Hence we conclude that there is a wide margin between the limit set to the speed of 
transmission by the gradual diminution of the received signals and that set by their inter- 
ference. 
If interference could be prevented by any change in the signals, it was clear that the 
capabilities of any given cable would be increased at least fourfold. 
Reverse currents have been much advocated, on good authority, as a means of greatly 
increasing the rate of signalling, and their effect was therefore examined, although it 
was not thought probable that the interference would be diminished by their use. By 
“reverse currents” the use of alternate positive and negative currents is meant. The 
negative current substituted for the earth-contact is supposed by its advocates in some 
way to clear the line and prepare the way for a positive signal. 
The connexions used during the experiments on “reverse currents” are shown in 
fig. 4, Plate XLIX. The first contact sent a positive, and the second contact a negative 
current through the line. 
The effects of single and reverse currents were directly compared on 1165 knots of 
cable. Tables VIII. and IX. show observations of the arrival-curve and signals when 
a positive current from 72 cells and an earth-contact were used. Tables X. and XI. 
show a similar set of observations when a positive current from 42 cells and a negative 
current from 30 cells were used. By keeping the same total number of cells in each 
