92 THE SCIENTIFIC PAPERS OF 



light on the remarkable changes to which gutta percha is subject. 

 Enough, however, can be shown to prove the importance of a 

 uniform and well-devised system of electrical tests, to be carried 

 on during the manufacture, shipment, laying, and subsequent 

 working of submarine cables. 



ELECTRICAL TESTS. The following system of tests was adopted 

 in reference to the Malta and Alexandria (formerly the Falmouth 

 and Gibraltar) cable : The covered strand of conducting wire, 

 in lengths of one nautical mile, was placed for 24 hours in tanks 

 filled with water maintained at 75 Fahr. They were then 

 removed into one of Reid's pressure tanks, containing water 

 of the same degree of temperature. The coils of wire under 

 operation, being by this time heated throughout to the above- 

 named temperature, were tested both for conductivity and insula- 

 tion ; and the result expressed in units of resistance, noted down 

 opposite the number of reference peculiar to each coil. A pres- 

 sure of GOO Ibs. per square inch was thereupon applied, and the 

 same electrical tests were repeated. Before the coil under ex- 

 amination was approved, it was required that the copper resistance 

 should not exceed 3'5 Siemens units of resistance, or possess 

 80 per cent, of the conductivity of chemically pure copper ,- and 

 that the gutta percha resistance per knot, at 75 Fahr., should 

 not exceed 90 millions of units, which also corresponds to about 

 80 per cent, of the highest insulation that can be attained with 

 the best gutta percha of commerce. It was further required, that 

 the insulation should improve when the pressure was applied, which 

 is invariably the case if the coatings are sound. The approved 

 coils of insulated conductor were transferred to the cable works 

 at Greenwich, where they were kept submerged in tanks until 

 the moment when they were required for the sheathing machine. 

 The sheathed cables were coiled into large tanks, where they 

 were intended always to be covered with water ; but, owing to 

 some defect in the construction of the tanks, this regulation could 

 only be partially carried into effect. It was also intended, in the 

 first instance, that the ships should be provided with water-tight 

 tanks, to receive the cable during the outward voyage ; but owing 

 to the passive resistance with which every deviation from previous 

 routine is usually met, these tanks were not provided, until the 

 heating of the cable on board the steam ship Queen Victoria had 



