GRAPPLING FOR THE SEA-SERPENT 157 
of electric potential existing between stations widely separat- 
ed on the earth’s crust; this engenders irregular earth currents 
in the cable. Varley overcame this difficulty by an ingenious 
device. Such problems had never been anticipated by the 
early promoters of the cable. 
Many improvements by the cable companies have been 
made in recent years. A modern permalloy cable, containing 
a single copper wire, uses only twenty-five milliamperes of cur- 
rent. ‘This is one twenty-five-thousandth of the amount neces- 
sary to ring a doorbell. An ocean cable is now laid in a furrow 
plowed in the ocean bed by a deep-sea plow. Thus it is pro- 
tected against icebergs, anchors, and fishing tackle. Earth- 
quakes, however, still snap cables occasionally. A modern 
cable costs about $2000 a mile to manufacture; the upkeep 
for repairs and maintenance is also high. 
