2. _The resistance of a conducting wire is inversely proportional to the area of its cross section_, and therefore in the usual round wires _is inversely proportional to the square of its diameter._ Ordinary telegraph wire is about 1/6th of an inch thick; a wire twice as thick would conduct four times as well, having four times the area of cross section; hence an equal length of it would have only 1/4th the resistance. 3. _The resistance of a conducting wire of given length and thickness depends upon_ the material of which it is made--that is, upon the specific resistance _of the material_. =Conductivity.=--This is _the inverse of resistance_. The term expresses the capability of a substance to conduct the electric current. If the symbol Y represent the conductivity of a substance, and I the current then: I/Y = its resistance; and if R represent the resistance of a substance, then I/R = its conductivity. Good conductors of heat are also good conductors of electricity. =Specific Conductivity.=--The figure which indicates the relation between one substance and another as to their capacity to conduct electricity is called _specific_ or _relative conductivity_. Taking the specific conductivity of silver as 100, that of pure copper is 96. The specific resistance of a substance is the reverse of its relative conductivity. The specific resistance of a metal is generally expressed in millionths of an ohm as the resistance of a centimeter cube of that metal between opposite sides. The following table gives the data for a few metals: Specific Resistance Specific Substance. in Microhms. Conductivity. Silver 1.609 100. Copper 1.642 96. Gold 2.154 74. Iron (soft) 9.827 16. Lead 19.847 8. German Silver 21.470 7.5 Mercury (liquid) 96.146 1.6 The specific resistance of copper is therefore: 1.642 / 1,000,000 ohms, or 1.642 microhms.[6] =Divided Circuits.=--If a circuit be divided, as in fig. 83, into two branches at A, uniting again at B, the current will also be divided, part flowing through one branch and part through the other. _The relative strength of current in the two branches will be proportional to their conductivities._ This law will hold good for any number of branch resistances connected between A and B. Conductivity is, as shown before, the reciprocal of resistance.