480 



SUBMARINE CABLE LAYING AND REPAIRING. 



resistances by the closing and opening of the battery key. The 

 following relation by inverse proportion then holds : — 



F h 



whence 



a" 



where F is the capacity of the cable and /is the capacity of 

 the condenser. Lord Kelvin (then Sir "William Thomson) one 

 year later referred to •' De Sauty's beautiful method " as a very 

 useful one on short cables, but painted out that it was not 

 applicable to long cables. 



I End free 



Fig. 2r33.— De Santy's Capacity Test. 



Kelvin's Mixed-Charge Test. — This method is well adapted 

 to the measurement of capacity on long cables. It is usually 

 taken with the slides as in Fig, 264. An ordinary key (not 

 shown) is connected in the battery circuit, so that the current 

 is only on the slides as required. The condenser and cable 

 are connected respectively to two levers of the mixing key 

 which when at rest make contact with the upper studs, so con- 

 necting the condenser and cable together. When the levers 

 are put into contact with the lower pair of studs (by pressing 

 down K) the condenser is connected to one end of the slides and 

 the cable to the other. As the opposite surfaces of the con. 

 denser and cable and the movable contact of the slides are to 

 earth, it follows that when the levers touch the lower studs the 

 cable and condenser are charged at potentials depending upon 

 the position of the slides. After charging, the key K is re- 

 leased, so connecting the cable and condenser. If the 



