=266. Resistance of Conductors.=--With an ammeter one may study the change produced in the amount of current flowing in a wire when a change is made in the wire conducting the current. For example, if one measures with an ammeter the current flowing from a dry cell through a long and then through a short piece of fine copper wire, it will be seen that less current flows when the long piece is used. That is, the long wire seems to hinder or to _resist_ the passing of the current more than the short piece. In other words, the long wire is said to have more _resistance_. The resistance of a conducting body is affected by several conditions. (a) It is _directly_ proportional to the _length_ of the conductor, one hundred feet of wire having twice the resistance of fifty feet. (b) It is _inversely_ proportional to the _square of the diameter_; a wire 0.1 inch in diameter has four times the resistance of a wire 0.2 inch in diameter. (c) It differs with different substances, iron having about six times as much as copper. (d) It varies with the temperature, metals having greater resistance at a higher temperature. Since silver is the best conductor known, the resistances of other substances are compared with it as a standard. The ratio of the resistance of a wire of any substance as compared to the resistance of a silver wire of exactly the same diameter and length is called its _relative_ resistance. Purified substances arranged in order of increasing resistance for the same length and sectional area (Ayrton-Mather) are given on p. 294. Silver annealed 1.00 Copper annealed { from 1.04 { to 1.09 Aluminum annealed 1.64 Nickel annealed 4.69 Platinum annealed 6.09 Iron annealed 6.56 German Silver { from 12.80 { to 20.20 Mercury 63.30 Nichrome 67.50 Carbon { from 2700.00 { to 6700.00 =267. The ohm, the unit of resistance=, is defined by international agreement as follows: _An ohm is the resistance of a column of pure mercury, 106.3 cm. long with a cross-section of a square millimeter and at a temperature of 0°C._ It should be noted that each of the four conditions affecting resistance is mentioned in the definition, viz., length, cross-section, material, and temperature. Since it is inconvenient to handle mercury, _standard resistance coils_, made of an alloy of high resistance are used in comparing and measuring resistances. A piece of copper wire No. 22 (diameter 0.644 mm.) 60. 5 ft. long has a resistance of 1 ohm. See table p. 296. The resistance of some telephone receivers is 75 ohms, of a telegraph sounder, 4 ohms, of a relay 200 ohms.