WILLIAM .S7/-:.I//-:.Y.V. l-\R.S. 15 



INSULATION RESISTANCES OF CABLES. 



The first Atlantic cable is represented by 12, that is to say, the 

 electrical resistance of the insulating coating of a mile of the cable 

 was equal to 12 millions of Siemens units; the Red Sea, by 30; 

 the Toulon and Algiers, by GO ; the Malta and Alexandria, by 120 ; 

 the Bona and Marsala, by 350 ; and the new Atlantic, by 400, the 

 latter being, however, more thickly covered ; so that you see 

 enormous strides have been made without any change of material, 

 simply by increased perfection in the manufacture. But it must 

 also be attributed to the application of proper tests during every 

 portion of the manufacture tests applied by the engineer who is 

 responsible for the work, and who insists upon a certain standard. 

 In the list to which I have referred, we see that there is a very 

 marked improvement in the case of the Malta and Alexandria line. 

 It may, indeed, be said that this was the first line that was ever 

 made under a proper system of tests. The system then adopted 

 and carried out under my immediate charge (by order of the 

 British Government), has since been applied to other cables with 

 only some modifications of details ; and with your permission, I 

 will give you a short account of what those tests were. 



It was determined in the first place that all tests should be 

 referred to a single standard of comparison, this being the resist- 

 ance of a column of mercuiy of 1 millimetre sectional area, and 1 

 metre length at freezing point, which standard is now generally 

 known as Siemens unit, having been first proposed by my brother, 

 Dr. "Werner Siemens, of Berlin. Instead of estimating the insula- 

 tion of the gutta-percha covering by the deflection of a galvano- 

 meter, as had previously been done, its conductivity per knot of 

 length was expressed in these units, and values were obtained 

 which were independent of the galvanometer and other testing 

 instruments employed, and admitted of direct comparison between 



