20S Prof. W. Thomson and Mr. F. Jetikin on the True and 



well as on the dimensions of the conductor, the gutta perch a, 

 and the outer iron sheath of the cable. The magnetic properties 

 of the iron sheath must greatly influence the false discharge ; and 

 it would be interesting to compare the discharge from a plain 

 gutta-percha-covered wire coiled under water with that from an 

 iron-sheathed cable. 



The following set of experiments, the last which we at present 

 adduce, illustrate the influence of less or greater intervals of time 

 during which the near end of the cable remains insulated, after 

 removal from the battery but before application to earth through 

 the galvanometer coil. 



Table IV. — 455 nautical miles of Alexandria and Candia Cable, 

 remote end kept to earth. Battery of 72 cells Daniell's. 



Experiment. 





Throw of needle 

 by true dis- 

 charge. 



Recoil, if any, 



and throw by 



false discharge. 



No. 

 1 

 2 

 3 

 4 



f# 5 ►.. ; 



• 6 





3 left 



S:: 



o" „ 

 o „ 



27 right 

 26 „ 

 20i „ 

 14 „ 

 17 „ 

 4 „ 



Key pressed down as usual . 

 Key pressed very gently . . . 

 Key held 5 seconds half-way. 



„ 10 



„ 15 



In order to detect whether there might not have been "a 

 slight hesitation in these three last instances, a much more deli- 

 cate instrument was taken, but no such hesitation could be de- 

 tected." These results are very remarkable, especially as regards 

 the duration of th<» electro-magnetic influence. If the conductor 

 of the cable w» re circumstanced like that of a common electro- 

 magnet, and had no sensible electrostatic capacity, the " mecha- 

 nical value* of the current in it " at the instant of the connexion 

 between its near end and the battery being broken, would be 

 spent in a spark, or electric arc of sensible duration between the 

 separated metal surfaces. But in the cable, the electrostatic capa- 



* .See a paper " On Transient Electric Currents," by Prof. W. Thom- 

 son, Phil. Mag. June 1853, where it is shown that, like the mechanical 

 value of the motion of a moving body, which is equal to half the square 

 of its velocity, multiplied by its mass, the mechanical value of a cur- 

 rent at any instant, in a coiled conductor, depending on electro-magnetic 

 induction, is equal to half the square of the strength of the current through 

 it, multiplied by a constant which the author denned as the "electro- 

 dynamic capacity of the conductor," and which he showed how to calculate 

 according to the form and dimensions of the coil. Additional explanations 

 and illustrations will be found in NiehoFs ' Cyclopaedia of Physical Science,' 

 second edition, 1859, under the heads " Magnetism — Dynamical Relations 

 of," and " Electricity — Velocity of." 



