Pure annealed copper has a specific gravity of 8.89 at 60° Fahr. One cubic inch weighs .32 pound; its melting point is about 2,100° Fahr.
Good hard drawn copper has a tensile strength of about three times its own weight per mile length. Thus, a number 10 B. & S. gauge copper wire, weighing 166 lbs. per mile, will have a breaking strength equal to approximately 3 × 166 = 498 lbs.
Iron Wire.—This kind of wire is largely used for telegraph and telephone lines, although it is rapidly being replaced by copper in long lines.
There are three grades of iron wire:
1. Extra best best (E. B. B.) which has the highest conductivity and is the nearest to being uniform, in quality, being both tough and pliable;
2. Best best (B. B.), which varies more in quality, is not so tough, and is lower in conductivity. It is frequently sold as E. B. B.;
3. Best (B.), which is the poorest grade made, being more brittle, and lowest in conductivity. Iron wire should be well galvanized.
German Silver Wire.—German silver is an alloy consisting of 18 to 30% nickel, and the balance about four parts copper to one part zinc. It is very largely used as a resistance material in making resistance coils, and is sold in the form of wire, and strip. The resistance of this wire varies with its composition.
The resistance of the 18% alloy at 25° C. is 18 times that of copper, and of the 30% alloy about 28 times that of copper.
The safe carrying capacity of the wire in spirals in open air for continuous duty is such that the circular mils per ampere varies from about 1,500 in No. 10 wire to about 475 in No. 30. For intermittent duty the capacity is twice as great.
Standard of Copper Wire Resistance.—Matthiessen's standard for resistance of copper wire is as follows: A hard drawn copper wire one meter long, weighing one gramme, has a resistance of .1469 B. A. unit at 32° Fahr. Relative conducting power: silver, 100; hard or un-annealed copper, 99.95; soft or annealed copper, 102.21.
A committee of the Am. Inst. Electrical Engineers recommends the following form of Matthiessen's standard, taking 8.89 as the specific gravity of pure copper: A soft copper wire one meter long and one millimeter in diameter has an electrical resistance of .02057 B. A. unit at 0°C.[2] From this the resistance of a soft copper wire one foot long and .001 in. in diameter (mil-foot) is 9.72 B. A. units at 0°C.
For every degree Fahr., the resistance of copper wire increases .2222%. Thus a piece of copper wire having a resistance of 10 ohms at 32°, would have a resistance of 11.11 ohms at 82°.
Relative Conductivity of Different Metals and Alloys.
(According to Lazare Weiler.)
| Pure silver | 100 | |
| Pure copper | 100 | |
| Alloy, ½ copper, ½ silver | 86 | .65 |
| Telephonic siliceous bronze | 35 | |
| Pure zinc | 29 | .9 |
| Brass with 35% zinc | 21 | .5 |
| Swedish iron | 16 | |
| Pure platinum | 10 | .6 |
| Copper with 10% nickel | 10 | .6 |
| Pure lead | 8 | .88 |
| Pure nickel | 7 | .89 |
| Phosphor-bronze, 10% tin | 3 | .88 |