988 ME. E. JENKIN’S EXPEEIMENTAL EESEAECHES ON THE TEANSmSSION 
Certain modifications of the usual signals were by these means discovered which 
materially increased the rate of legible transmission. 
These experiments were all made on one cable, and do not therefore refer to the 
effects produced by a change of dimensions or materials ; they will now be described in 
detail, and the connexion of the results with the mathematical theory will then be 
mentioned. 
The Eed Sea cable only was used in all the experiments ; the external diameter of 
the insulated wire was 0 ‘34 inch; the gutta percha weighed 212 lbs. per knot; the 
conductor was a copper strand of seven wires, and weighed 180 lbs. per knot. The 
extreme diameter of the strand was about OTOSinch, and the ratio of the external to 
the internal diameter of the gutta percha sheath may be taken as 3'4. The resistance 
of the conductor per knot was 25 x 10^ British absolute units. 
The first experiment was made on July 26th, 1859, with 2168 knots disposed in ten 
coils, each about 26 feet in diameter, and held in a dry iron tank ; the cable A CX, the 
battery B, a Morse key M, a galvanometer G, and the two earth-plates E E were con- 
nected as in fig. 1, Plate XLIX. 
The effect at the end X produced by connecting the cable at A alternately with the 
battery and the earth-plate by means of the key was observed on the galvanometer G, 
being Professor Thomson’s marine galvanometer. 
The suspended magnet m of this galvanometer carried a little mirror n reflecting the 
image of a flame B on to a scale A about 26 inches off, as shown in fig. 2. The zero 
of the scale was in the middle, and each division was equal to -^-th of an inch. The 
coil C is shown in section, and the little lens L in its position in front of the mirror. 
The deviations of the spot of light from the centre measured on the scale were sensibly 
proportional to the strength of the current causing the deflection, when the deviations 
did not exceed 200 divisions. 
The little magnet was powerfully directed by a fixed external steel magnet N S. Tire 
weight of the moveable magnet and mirror was about grain ; the inertia of the 
moving parts was consequently so small, and the directing force so great, that the varia- 
tions of a rapidly changing current were accurately represented by the movements of 
the spot of hght along the scale. When the Morse key (fig. 1) was pressed down, the 
spot of light remained apparently motionless for a short but sensible time, then shot 
along the scale, moving rapidly at first, but gradually losing speed, until at last it 
moved very slowly to a maximum deviation, at which it remained quite still; these 
movements truly showed the gradual change of the received current from nothing to a 
maximum, a change requiring fifty seconds for its completion. During the latter part 
of this time the movement of the spot was slow enough to allow the moment at which 
it passed any given division of the scale to be pretty accurately fixed. Two observers 
refers to the speed with which certain changes in the received current called signals can be made to follow 
each other, and not to the velocitj of propagation of the electric current. 
