SCIENCE AND PRACTICE. 367 



sufficiently well with the corresponding observations with the 

 core of the Malta-Alexandria cahle, to render it highly pro- 

 bable that the per-centage improvement in the resistance 

 under pressure is independent of the thickness of the in- 

 sulator, as well as of improvement in the quality of the 

 material. 



81. Electrification. In the earliest days of cable-making 

 attention was called to the fact that, when a current is kept 

 on an insulated wire, the insulation resistance increases with 

 the time, but not proportionably to it. 



The late Dr. Esselbach made some elaborate experimental 

 investigations to determine empirically the nature of the 

 curve, during the time he officiated as government electrician 

 to the Malta- Alexandria cable, and had prepared himself to 

 prosecute his researches on this subject with the Persian 

 Gulf line, of which he held the position of general- super- 

 intendent. It is much to be regretted, however, that his 

 untimely death has deprived the science of the most valuable 

 part of that which he had already accomplished in illustra- 

 tion and explanation of the phenomenon. 



If we take a wire, insulated with gutta-percha, and con- 

 nect the pole of a battery to one end of it, we find that the 

 observations of the current, after stated periods of time, will 

 give us a curve represented by the line a b, Fig. 169, in 

 which the strengths of current are the ordinates, and the times 

 the abscissae. It is evident that the decreasing current 

 observed is due to two causes the one to leakage through 

 the material, and which is the proper insulation current, and 

 the other to electrification. A glance at the line which we 

 obtain will suffice to make it evident that the curve is 

 assymptotical, nearing the axes of the system in both direc- 

 tions. From this it is obvious that the current which we 

 measure is never the true insulation- current 1, but always 1 

 plus some function of the time, although the curve after an 

 hour or so approaches very near to the line of the true 

 insulation- current. 



There is but one conclusion to be drawn from this phe- 

 nomenon : it is, that a cable takes an infinitely long time to 



