48 THE SCIENTIFIC PAPERS OP 



the wire in several coatings, closely adhering to one another ; air 

 bubbles should be strictly avoided, and the concentricity of the 

 entire coating be insured by the use of very perfect machinery and 

 strict avoidance of stoppages during the process of covering, to 

 prevent a softening of the several coatings by heat. 



Great improvements have of late been effected in the process of 

 covering electric conductors with gutta-percha and intermediate 

 layers of a compound called " Chatterton's mixture," which may be 

 estimated by the fact that the covering of the Rangoon and Singa- 

 pore cable, now in process of manufacture, insulates fully ten 

 times better than the covering of the Red Sea and India cable did 

 before it was laid. 



This marked improvement is due to the greater care taken by 

 the Gutta-Percha Company in the manufacture, under a system of 

 stringent electrical tests, which we are charged by the British 

 Government to apply. The objects of these tests is, in the first 

 place, to ascertain the specific conductivity of each mile of the 

 copper conductor, in order that all below a certain fixed standard 

 may be rejected. 



An inquiry into the extraordinary variations in the conductivity 

 of the copper of commerce has been made the subject of a careful 

 investigation by Dr. Mathiessen for the British Government, which 

 will probably shortly be published. 



In practice we find that the best selected copper employed for 

 telegraphic conductors varies as much as twenty per cent, in its 

 conductivity and that the purer copper conducts the best. 



The conductivity tests of each mile of an. insulated conductor 

 are very necessary, not only to reject the faulty material but also 

 to obtain a complete record of the conductivity of each portion of 

 the cable when completed, without which it is not possible to 

 determine afterwards by galvanic tests and calculations the precise 

 position of a fault. 



The more difficult and most important tests are those of the 

 conductivity of the insulating material of each mile of insulated 

 conductor, for it is not sufficient to find out any palpable fault or 

 leakage but to appreciate eccentricities, cavities, or other minor 

 defects in the coating, and to reject what falls below the standard 

 of conductivity of the insulating material in its most perfect 

 condition. 



