358 ABSTRACTS OF SCIENTIFIC PAPERS 



and I the length. The series quoted above applies to the case of 

 perfect insulation : another expression is given, applicable to wire 

 whose insulation is imperfect. 



Guillemin's experiments had apparently been made without regard 

 to the theory of Thomson, but with reference to an equation for the 

 establishment of the electric current given by Ohm as early as 1827, 

 where an unknown coefficient appears instead of the quantity c, the 

 place of which in the theory was pointed out by Thomson. The results 

 of Guillemin's experiments were however, as was to be expected, in 

 accordance with the completed theory. His method of determining 

 the curve of arrival was to cause a periodic variation of potential to 

 occur at the sending end, by means of a revolving cylinder which 

 connected the line there alternately to battery and to earth at regular 

 short intervals. The circuit was a loop line with its ends near 

 together, and the same mechanism which sent the currents was 

 employed to connect the receiving end with a galvanometer for an 

 excessively short interval once for every current sent into the line. 

 The successive impulses thus given to the galvanometer needle pro- 

 duced a steady deflection which served to measure the potential at 

 the receiving end at the particular epoch in the curve of arrival 

 when momentary connection with the galvanometer was made. 

 Professor Jenkin proceeds, using the data supplied by Guillemin's 

 results, and assuming a value for the resistance of the line, to apply 

 Thomson's formula to calculate the capacity c, and finds for it a 

 value nearly twice as great as the calculated capacity of a perfectly 

 insulated wire suspended at a uniform distance from a conducting 

 1 



plane, namely, ~~ 4h where h is the height and d the diameter of 

 2 log =' 



Cv 



the wire. He ascribes the difference partly to the approach of the 

 posts to the wire, partly to the capacity of the insulators, and pos- 

 sibly to the effects of polarisation at the points of support. He adds 

 that an additional retardation (which has the effect of making the 

 capacity as calculated for M. Guillemin's results appear larger than 

 its actual value) is produced by the electrodynamic induction from 

 wire to wire. He concludes with an estimate of the rate at which 

 signals can be sent by instruments such as the Wheatstone automatic 

 transmitter, where a limit of speed is set by the retardation discussed 

 in the paper. 



XVIII. Lectures on the Construction of Telegraph Lines. Royal 

 Engineers Institute, Chatham, 1863. 



