418 
POPULAE SCIENCE EE VIEW. 
other, which, though reaching to the other side of the Atlantic 
and back again— a distance of some 3,500 miles, should offer 
less resistance to the passage of an electric current than any 
other possible route by which it could traverse the few feet or 
inches by which the two poles were separated. For this pur- 
pose it was necessary, in the first place, to diminish as much 
as possible the resistance which would be encountered by a 
current passing from one pole of the battery to Newfound- 
land and returning thence to the other pole ; and, in the second 
place, to make as great as possible the resistance offered to 
the passage of the current from one pole to the other by any 
shorter path. 
The first of these objects was effected by extending between 
Yalentia and Newfoundland a copper wire, — pure copper 
having the smallest electrical resistance of any substance 
known, with the exception of silver. When the Irish end of 
this wire is connected with the positive pole of the battery, 
and the other end of the wire and the negative pole of the 
battery are at the same time each of them connected with a 
large metallic plate (called an “ earth-plate ”), either buried 
in the ground or sunk into the sea, the current starting from 
the positive pole passes along the wire to Newfoundland, then 
through the Newfoundland earth-plate into the ground or into 
the ocean, and so back through earth, or water, or both, to the 
Yalentia earth -plate and the negative pole of the battery. 
This conductor is formed of a strand of seven wires, each of 
them 0*048 inch in diameter, and therefore together equivalent 
to a single wire of nearly 0*144 inch diameter. Its length, as 
laid, is about 1,858 knots, and each knot has an electrical 
resistance at 24° C. (or 75° Fahr.), equal to 4*272 times the 
unit or standard of resistance adopted by the Committee of 
the British Association on Electrical Standards, and known as 
the “ British Association-Unit.^ Consequently, the resistance 
of the whole length of the conductor may be taken in round 
numbers as equal to 7,500 B. A. -units, allowing for a diminu- 
tion of resistance caused by the low temperature of the bottom 
of the Atlantic. To most persons this number would certainly 
convey no very definite idea ; but to an electrician it repre- 
sents a resistance to the passage of electricity which must be 
greatly exceeded by the resistance, not only of any one other 
path, but of all other possible paths taken together, by which 
the current could pass from one pole of the battery to the 
other. 
To appreciate what is involved in this last statement, we 
must remember that the superficial area of the copper con- 
ductor, according to the dimensions already given, is about 
425,000 square feet, and that this large surface is surrounded 
