THE 

 LONDON, EDINBURGH, and DUBLIN 



4 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



SEPTEMBER 1867. 



XXI. On testing Telegraph Cables during the process of Sheath- 

 ing. By Louis Schwendler*. 



A TELEGRAPH cable, when complete, should be "electrically 

 normal'" i. e. its resistances of conductor and insulator 

 should correspond exactly with those of the different lengths of 

 which it is composed, and which were tested before being joined 

 together during sheathing. 



The excellent method first suggested and applied by Messrs. 

 Siemens in testing the Malta-Alexandria cable, gives us both the 

 resistances of insulation and conduction for equal lengths of core 

 at a standard temperature and pressure, expressed in the same 

 unit, the period of time which elapses between connecting the 

 battery to the cable and taking the reading being always the same. 

 It has occurred to me, nevertheless, that at present we are not 

 in a position to make a profitable use of these valuable tests to 

 decide the normal condition of a cable satisfactorily. As regards 

 the conductor, we are indeed able to do so; for not only can we 

 measure its resistance very exactly, but we are also able to cal- 

 culate the conductor resistance of the whole cable from those of 

 its different parts ; and if this calculated resistance (the tempe- 

 rature being taken into account) coincides with the measured 

 resistance, we may consider the cable "normal in its conduc- 

 tivity" 



But the same cannot be said of the insulation, which, however, 

 is of greater importance. The insulation resistance of a cable 



* Communicated by the Author. 

 Phil. Mag. S. 4. Vol. 34. No. 229. Sept. 1867. N 



