OF ELECTEIC SIGNALS THEOEGH SEBMAEINE CABLES. 
995 
case, the difference of potentials between the two sources of electricity alternately 
in connexion with the line was maintained nearly equal. Tables VIII. and IX. are 
similar to those already described. 
The observations in Table X. were made in almost the same manner as those in 
Tables VIII. and IX., but the following description may make the meaning of the 
entries more distinct. 
The Morse key when untouched left the negative battery connected with the line, and 
the spot of light then stood at 113 divisions to the left of zero ; when the key was 
permanently pressed down, the spot stood at 157 divisions to the right of zero. All 
deviations to the right are entered as positive, those to the left as negative. Four 
seconds after the key was pressed down, the spot passed the 100th division to the right;, 
four seconds after the key was released, it passed to the 60th division on the left. In 
the first case it had traversed 213 divisions in four seconds, in the second case 217 divi- 
sions, or 78’8 and 80‘3 per cent, respectively of the sum of the two deflections 157 
and 113. 
On examining the last columns of Table X. we find that in any given time after 
moving the key, the spot traversed an equal length of the scale whether the change was 
from positive to negative or from negative to positive, or, in other words, the rate of 
decrease in the current received at X after contact had been made with the negative 
battery at A for a given time was the same as the rate of increase observed after making 
contact at A with the positive battery for an egual time., — a conclusion exactly analogous 
to that arrived at when single currents were used. 
Moreover, comparing the last columns of Tables VIII. and X., we find that the spot 
traversed the same distance in the same time in each case, and therefore that the rate 
of increase and rate of decrease, caused by reverse currents, is exactly the same as that 
observed when single currents are used. 
The arrival-curves from the two Tables are shown in figs. 5 & 6, Plate XLIX. 
The curves are identical in shape, and differ only in their position relatively to the 
zero-line. 
Thus the ratio between the ordinates of the arrival-curve is the same whatever the 
two sources of electricity at A, or the potential of the earth at X may be. 
Hence we conclude, 
1st. That the absolute change in the strength of the received current during a given 
time after a change in the contact at A, is not influenced by the potential maintained 
at X, but is simply proportional to the difference of potentials or electromotive force 
between the two sources of electricity alternately connected with the cable at A. 
2nd. That the rate of change is independent of the potentials both at A and X. 
It was now clear that the alternation of negative and positive currents could have no 
effect on the rate of signalling, but to avoid aU cavil a few experiments were made with 
the usual signals. 
The results are given in Tables IX. and XI. The signals sent by the reverse currents 
MDCCCLXII. 6 u 
