206 Prof. W. Thomson and Mr. F. Jenkin on the True and 

 Table II. — 455 nautical miles of Alexandria and Candia Cable. 



Remote end of cable kept 



First throw of 

 needle. 



Recoil or second 

 throw. 



Excess of recoil 

 above first throw. 





2£- right 

 5 » 



in „ 



16* „ 



3^ „ 



24£ left * 



22 „ 



18^ „ 

 21 „ 



not ohserved 



24 „ 



22 

 17 



7 



H 



20^ 



2. To earth through 10"] 



German miles, resist- \ 

 ance units* | 



3. To earth through 50 „ 



4. To earth through 90 „ 



6. To earth direct, and j 

 key " pressed very I 

 sharp home " J 



If the whole duration of current, with or without reversal, 

 through the galvanometer coil had been infinitely small in com- 

 parison with the natural time of oscillation of the needle (which, 

 reckoned in one direction, was about 4^ seconds), the recoils 

 would have been sensibly equal to the first throws in the con- 

 trary direction, being only less by the effect of resistance of the 

 air, &c. to the motion of the needle. Hence the numbers in 

 the last column of the preceding Table prove that at some inter- 

 val of time, not incomparably less than 4^ seconds, after the 

 first motion of the needle, there was a current through the gal- 

 vanometer coil opposite in direction to that which produced the 

 first or right deflection, in each case except No. 5, or that in 

 which the remote end of the cable was insulated. It may be 

 safely assumed that the conductors used in cases 2, 3, and 4 to 

 give the stated resistances between the remote end of the cable 

 and the earth, exercised no sensible electro-magnetic influence, 

 and held no sensible charge, in the actual circumstances; and it 

 is interesting to see how the greater the resistance thus intro- 

 duced, that is to say the more nearly the remote end is insulated, 

 the greater is the first throw (due, as explained above, to true 

 discharge), and the less is the excess of the recoil above it. 



This excess, shown in the last column of the Table, exhibits 

 the effect of the electro-magnetic induction from coil to coil 

 which stops short the true discharge, and produces after it a 

 reverse current constituting the "false discharge." The fol- 

 lowing experiments, performed by Mr. Jenkin on the 19th of 

 April, 1859, on different lengths of the Red Sea cable, illustrate 

 the relations between true and false discharge. 



* The resistance of this unit was found by experiment to be equal to about 

 lf)0 x 10 f ' British absolute units of feet per second, or to 6} nautical miles 

 of the Alexandria and Candia cable, or to4'39 of the Dardanelles, or to 7'44 

 of the Red Sea. 



