Electric current metal (ideal) resistor (or other ohmic device): where formula_6 is the current, measured in amperes; formula_26 is the potential difference, measured in volts; and formula_27 is the resistance, measured in ohms. For alternating currents, especially at higher frequencies, skin effect causes the current to spread unevenly across the conductor cross-section, with higher density near the surface, thus increasing the apparent resistance. The mobile charged particles within a conductor move constantly in random directions, like the particles of a gas. (More accurately, a Fermi gas.) To create a net flow of charge, the particles must also move together with an average
Spreading resistance profiling least formula_6 thick. With the aid of calibration resistivity standards, formula_3 can be determined at each probing by the probe pair. A bias of 5mV is applied across the probe tips. The measured resistance can range from 1-ohm to one billion ohms. A "log R" amplifier or electrometer is used to measure the resistance. The modern SRP has two tungsten carbide probe tips placed about 20 um apart. Each tip is mounted on a kinematic bearing to minimize "scrubbing". The probes are lowered very gently onto a beveled piece of silicon or germanium. Although the loading of the probe tips