What is electrical resistivity?
Dr. Don Stierman
Consider a wire with length L and cross-sectional area A.
The electrical resistance (R - recall Ohm's Law, V = IR) is directly proportional to L and inversely proportional to A. We could write the equation as
where ρ is a property of the material from which this wire is fabricated, a property called its electrical resistivity. Good conductors like copper and gold have very low resistivities, poor conductors like feldspars and glass have very high resistivities.
Electrical resistivity usually increases with temperature. So, although resistivity is a fundamental physical property, it is a variable physical property. In good conductors, electrical resistivity increases at high electrical current levels as loosely held electrons that carry the charge become scarce. Insulators can break down at very high voltages.
In geophysical exploration, charged particles are usually scarce near current electrodes. In measuring very low resistances, physicists use a 4 electrode method, measuring the current through a component while measuring the voltage drop across the component 'inside' the current contacts. Thus we cannot simply plug a volt meter into the ground and get a reliable resistance. We transmit an electrical current into the earth and measure the earth's response.