418 



Mr. F. Jenkin on the Retardation of 



for the length of 350 miles, and with the degree of insulation 

 given by good lines in bad weather. 



Table IV. 



Times after making con- 1 



tact,= - [ 



a J 



Calculated strength of cur- "\ 



rent with perfect insula- 

 tion for any strength and 

 length of current, maxi- 



mum being 10 j 



_.,, ..i fw=20,000 

 Ditto with I l= 350 



. ,m P 1 ei J ect in=40 l 0001 

 insulation. /= ' 350 | 



2 



4 



6 



10 



14 





0-64 



3-88 



6-46 



8-58 



9-99 





0-89 



4-85 



7-5 



9'4 



9-99 





106 



5-56 



8-15 



9-64 



9-99 





It is clear that the curves represented by these ordinates depart 

 still more from those given by the experiments ; and we may 

 therefore presume that the experiments were tried in fine weather, 

 when the insulation was much better than the above. 



The calculation of c, the electrostatical capacity per metre, may 

 be made as follows. 



Taking Table I., we find the mean value of a from the last 

 column, neglecting the first number, to be very nearly nineteen 

 and a half 10-thousandths of a second. 



The resistance of one kilometre of iron wire, 4 millims. in 

 diameter, is about 9*27 B. A. units, or say 92,700,000 absolute 

 electromagnetic ~^* This value is obtained from the value of 

 Messrs Digney's resistance- coils expressing one kilometre of iron 

 wire, and exhibited in the International Exhibition for 1862. 

 The value there given from Professor Weber's determination of 

 the metre-second is 100,800,000, and the value taken in the 

 present paper results from the more accurate determination of the 

 Committee on Electrical Standards. The resistance of one metre 

 of the wire is therefore taken as 9'27 X 10 4 in electromagnetic 

 units. 



To convert this into electrostatic measure, it must be divided 

 by (310,740,000) 2 (vide Report of British Association for 1863, 

 p. 159) ; hence 



k= 9-656 xl0 1 1? eqUal nearly t0 9 ' 6 X 10 ~ 13 ' 



Z= 570,000; and if we take a equal to 0*00194 second and 

 substitute these values in the equation given above, we obtain the 



2 ~l 



value ofc=-rs- -, -. =0'213. ThemeanvalueofafromTablell. 



kP log % 



