TRANSACTIONS OF THE SECTIONS. 251 



On the Retardation of Signals through long Submarine Cables. 

 By Fleeming Jenkin. 



This paper contains the result of experiments made on long submarine cables, at 

 the establishment of Messrs R. S. Newall and Co. 



Professor Thomson's theory* is confirmed by these experiments, which in- 

 deed were only rendered possible by the use of Professor Thomson's Patent Marine 

 Galvanometer. 



The deflections of the magnet in this galvanometer are read by means of a spot of 

 light, reflected by a mirror attached to the magnet, and brought to a focus on a 

 scale at about 22 inches from the mirror. The magnet and mirror together weigh 

 only 1£ grain, and a very small angular movement of the magnet causes the spot of 

 light to move over many degrees of the scale. 



By this instrument a gradually and rapidly increasing or decreasing current is at 

 each instant indicated at its true strength. 



When therefore this galvanometer is placed as a receiving instrument at the end 

 of a long submarine cable, the following phenomena are seen. At the moment of 

 completing the circuit through battery, cabl«, and earth at the sending end, no move- 

 ment of the spot of light occurs. In a second or less the spot begins to traverse the 

 scale, at first slowly, then rapidly, and again more slowly, until after perhaps a 

 minute a maximum is attained. 



The interval of time which elapses between the first completion of the circuit and 

 the arrival of the spot of light at various divisions of the scale was measured, and the 

 observations being thrown into a curve, have given what may be termed the " curve 

 of arrival " for various lengths. 



Instead of one continuous current, broken currents, such as form dots and dashes 

 in the Morse alphabet, were also sent with regularity by means of a metronome, and 

 the movements of the spot of light corresponding to the various signals observed and 

 delineated by curves. 



The following are the results of the observations : — 



1. The strength of the battery used does not affect the speed of transmission ; to 

 prove this, the curve of arrival was taken with 72 cells on a length of 2168 knots and 

 with 36 cells. The two curves coincided when drawn to scales proportionate to the 

 electromotive force of the two batteries. Thus is once more proved the fact that the 

 speed of electricity is independent of the power of the battery, the current always 

 reaching the same fraction of its maximum strength in the same time. 



2. The same curve represents the gradual increase of intensity in a current when 

 arriving, and the gradual decrease of intensity caused by putting the sending end of 

 the cable to earth. 



3. The curves of arrival, as obtained from lengths of 1000 to 2000 knots, agree in 

 general character with those given by Professor Thomson's formula. Some discre- 

 pancies appear, due probably to electro- magnetic induction between the coils, and also 

 in great measure to the varying resistance of the insulating cover, described in the 

 author's paper " On Gutta Percha as an Insulator." 



4. Whatever length of the cable was used for experiment, the amplitudes of oscil- 

 lation representing dashes, or A's or other letters, were found to bear a constant pro- 

 portion to the amplitudes representing simple dots sent at the same speed. This 

 proportion is, however, different for each amplitude. Thus on a length of 2191 knots 

 the speed of 15 dots per minute reproduced the same amplitude of oscillation as a 

 speed of 30 dots on a length of 1500 knots ; and the same relative speeds reproduced 

 the same oscillations for dashes, A's, &c. in the two lengths. The amplitude is in 

 this paper supposed always to be measured as a fraction of the maximum deflection 

 obtained by keeping the circuit completed till the spot of light comes to rest. 



5. The speed at which signals could be received on a relay, is easily perceived 

 when the groups of oscillations are graphically delineated. A certain constant am- 

 plitude of dot corresponds to this speed (vide § 4). The speed at which a given 

 amplitude of dot can be produced, varies inversely as the square of the length ; and 

 therefore the speed at which signals can be received by a relay varies also inversely 

 as the square of the length of the cable. 



* Proceedings of the Royal Society, May 1855, republished in the Philosophical Magazine. 



