SCIENCE AND PRACTICE. 373 



charge at the moment of discharging, remains unchanged, 

 whatever the current may be. If the current is small, as it 

 must be if the joint is good, the charge will be proportion- 

 ally small ; and it will increase as the current increases. In 

 shifting the condenser by an instantaneous movement from 

 the battery and joint to the galvanometer, no time is given 

 for any material part of the charge, at the moment in the 

 condenser, to neutralise itself through the dielectric ; and, 

 therefore, whatever is present shows itself upon the instru- 

 ment. 



The core, when joined up in convenient lengths, is coiled 

 carefully upon the drums and transported to the sheathing 

 works. Arrived there, if not wanted immediately to be 

 sheathed, it is stored away in water-tanks, and tested at in- 

 tervals, to ascertain if it has become injured, and to accu- 

 mulate data for subsequent calculations. 



84. Self-heating of Cables. During the manufacture of 

 the cable, also, its electrical conditions are ascertained at 

 regular intervals usually twice a day to make sure that 

 no injury has happened to the insulator ; or, in the event of 

 the cable being coiled in a dry tank, that it has not heated 

 spontaneously, and increased the temperature of the core. 

 This is seen by the increased copper resistance and lower 

 resistance of insulation. 



The cable destined in 1860 for submersion between Ran- 

 goon and Singapore, and subsequently laid in the Mediter- 

 ranean, was coiled in tanks in the yard at Morden Wharf, 

 Greenwich. While there, some of the tanks became leaky, 

 the weight of the cable bearing too heavily upon their 

 foundations ; and it was decided not to pump water over the 

 cables, because the iron rusted very quickly, and the oxide, 

 being washed off by every new supply of water, would soon 

 have reduced the sections of the wires. After some days' 

 exposure to the air in this half- wet condition, we found that 

 some of the cables showed signs of deterioration and a higher 

 copper resistance than was due to the temperature of the 

 surrounding air. The cable which appeared the most de- 



