Ohm demonstrated that there are no "perfect" electrical conductors through a series of experiments in 1825. Every conductor he tested offered some level of resistance.  These experiments led to Ohm's law.  Ohm's law of 1826 states that if the ambiant temperature remains constant, the current flowing through certain conductors is proportional to the potential difference(voltage) across it. In other words, current equals voltage divided by resistance. "I" is current, "V" is voltage, and "R" is resistance in the equation and diagrams.

I=V/R

Or  "V" (the voltage or the electomotive force - measured in volts) equals "I" (the current - measured in amperes) times "R" (the resistance - measured in ohms).  An ohm is the SI unit of resistance. It is the resistance between two points of a conductor under a certain set of circumstances. There must be a constant difference of potential (the work required to bring a unit of electric charge) of 1 volt applied between these two points, producing the conductor a current of 1 ampere.  The diagram below show exactly how Ohm's Law works with a common battery. The yellow light represents a bulb which is powered by the battery. The tan wire shows where the resistence occurs.

Resistance is property of any object or substance to resist or oppose the flow of an electrical current. The unit of resistance is the ohm. The abbreviation for electric resistance is R and the symbol for ohms is the Greek letter omega. For certain electrical calculations the reciprocal of resistance is used, 1/R, which is termed conductance, G. The unit of conductance is the mho, or ohm spelled backward, and the symbol is the reciprocal of omega.

In principle, it is relatively simple to measure the resistance of a strand of wire. Connect a battery to a wire of known voltage and then measure the current flowing through the wire. The voltage divided by the current yields the resistance of the wire. In essence, this is how your multimeter measures resistance. In making this measurement, however, we must ask two crucial questions.  How is the measured resistance related to some fundamental property of the material from which the wire is made?  How can we apply this relatively simple experiment to determine electrical properties of earth materials?

The problem with using resistance as a measurement is that it depends not only on the material from which the wire is made, but also the geometry of the wire. If we were to increase the length of wire, for example, the measured resistance would increase. Also, if we were to decrease the diameter of the wire, the measured resistance would increase. We want to define a property that describes a material's ability to transmit electrical current that is independent of the geometrical factors.

 
High values of resistivity imply that the material making up the wire is very resistant to the flow of electricity. Low values of resistivity imply that the material making up the wire transmits electricial current very easily.

The geometrically-independent quantity that is used is called resistivity and is usually indicated by the Greek symbol r. In the case of a wire, resistivity is defined as the resistance in the wire, times the cross-sectional area of the wire, divided by the length of the wire. Why did the chicken cross the circut? (just a test to see if you really are awake.)  The units associated with resistivity are thus, ohm - m (ohm - meters). The diagram bellow shows this equation as it would work with a common wire, represented by the tan cylinder.