VELOCITY OF ELECTRICITY. 



213 



This rate will carry electricity round the earth in about a twelfth of a second, 

 a rate Tuck never dreamed of when he promised to " put a girdle round the 

 earth in forty minutes." 



But it appeared from investigations subsequently made that it was not 

 possible to express the velocity of electricity with any certainty, and a 

 number of experiments were made as per the following table, with very 

 different results. Sir William Thomson and Faraday endeavoured to 

 account for these stupendous discrepancies, and the principles of retardation 

 of electricity were established. The differences are shown below : 



Nature of Velocity per 



Wire. Second. 



vv heatstone's Experiment in 1834 . . . . Copper . . 288,000 



Gonella and Fizeau . (Copper . . 111,834 



(Iron . . 62,130 



Mitchell ... . f . . . . .. Iron . . 28,331 



Walker ...,.,. . Iron . . 18,639 



Gould Iron . , 15.830 



Astronomers in Greenwich Copper . . 7,600 



Brussels Copper . . 2,700 



Result in Atlantic Cable, 1857 . . . . Copper . . 1,430 



1858 Copper . . ' 3,000 



To account for the comparatively low velocities of the cables, Faraday 

 proved that they act very much as a Leyden jar acts ; that is, it takes time 

 to fill, as it were, and to discharge them, the wire coating of the cable in air 

 acting like the outer coating of the jar or the water in the case of an immersed 

 cable, and the retardation observed is owing to resistance of conduction, and 

 depends upon the way in which the electrical impulses traverse the wire. 

 " There is a long, gradual swell, and still more gradual subsidence of the 

 electric current, and the length of time that elapses between the initial 

 impulse and the attainment of maximum strength, is proportional to the 

 square of the length of the line." 



The duration of the electric spark has been calculated at the - 2 4oro 

 part of a second, but Professor Tyndall regards this as the longest or nearly 

 the longest time it is perceptible ; the shortest time is almost inconceivable. 

 The brightest portion of a spark has been ascertained to last only forty-six 

 millionths of a second, and certain experiments were made to ascertain the 

 actual duration with various numbers of Leyden jars. It was discovered by 

 Messrs. Lucas and Cuzin, by an application of the Vernier, with batteries 

 consisting variously of two to eight jars, and obtained the following results* 



Duration in 



' No. of Jars. millionths of a Second. 



2 26 



4 

 6 



8 

 " So," adds the writer, 



. 41 

 . 45 

 47 

 the duration (of spark) increases in proportion 



with the number of the jars. It increases also with the striking distance, but 



* GANOT : Elements de Physique. 



