54 ANNUAL OF SCIENTIFIC DISCOVERY. 



strain upon it caused by an immense accumulation of ice, the "wir^ 

 itself remaining intact. 



Large wire is used only because of its superior conductivity ; 

 and it is obvious that a light wire is preferable in handling and 

 stringing, which can be done with less labor. 



Also, maintaining a superior conductivity and relative strength, 

 the lightness of this wire will admit of an average of at least 10 

 poles to the mile less than would otherwise be necessary. 



This reduction in the number of poles per mile will not only 

 conduce to economy in construction, but it will effect a decrease 

 of 25 per cent, or more in escape of the electric current. 



In stringing over the tops of buildings, stretches may be safely 

 made double the length of those taken with the ordinary telegraph 

 wire, and yet with less strain upon the insulators. 



Another point in its favor is the imperishable nature of copper, 

 which, in this wire, is the exposed metal ; the zinc coating of the 

 galvanized iron being deteriorated near the sea, and from the 

 effect of gases, etc., from chimneys, while copper will remain, 

 under such conditions, unimpaired. In fact, under all circum- 

 stances, the durability of the compound wire is greatly superior 

 to that of the galvanized wire in general use. 



In the construction of lines there are many cases in which the 

 expense of transportation of telegraph wire from the manufactory 

 to its destination is an item of considerable magnitude. With the 

 same or a much greater conductivity, as compared with galvanized 

 iron, the compound wire weighs materially less, with no disadvan- 

 tage whatever arising from its lightness. 



Referring again to conductivity, which has been the chief point 

 in the production of this wire, it will be observed that this ele- 

 ment may be largely increased without sacrificing strength, and 

 without recourse to an unwieldy medium for conduction. 



Increased conductivity admits of a reduction in battery power, 

 with a consequent decrease in the escape of electricity. Long 

 circuits are worked with greater facility, and the rains and the 

 fogs lose their power to prevent the passage of the electric current 

 where it should properly flow. 



The term '* relative strength " denotes the quotient obtained by 

 dividing the strain which would break the wire by its weight per 

 mile. 



The compound wire need have only about one-third the weight 

 of galvanized iron wire to be relatively stronger, and at the same 

 time to possess equal or greater conductivity. 



It is evident why this should be so, since the best commercial 

 copper possesses more than 6 times the average conducting ca- 

 pacity of galvanized iron wire ; and the steel which enters into 

 the compound wire has nearly 3 times the tensile strength of 

 galvanized iron wire of equal size. 



The relative strength of the steel which is used in the American 

 compound telegraph wire averages 7.47 ; that of the copper, 1.72 ; 

 while the average relative strength of galvanized iron wire, as 

 found by testing various samples of the best in the market, is 

 only 2.9. 



