The Resistance of Copper Wires at Very High Frequencies

At frequencies of the order of ${10}^7$ cycles the distributed capacity of single loops of wire may cause sufficiently unequal current distribution in the loop to account for large apparent discrepancies between observed and calculated resistances. For a given frequency, more uniform current distribution is gained by decreasing the size of the loop and simultaneously increasing the capacity of the tuning condenser. Curves are plotted with ratio of observed to calculated resistance as ordinate and condenser setting as abscissa. For No. 20 copper wire at 0.86×${10}^7$ cycles the ratio decreases to at least 1.05 as the current distribution along the wire is made more and more nearly uniform. For No. 16 oxide coated copper wire the ratio reduces to at least 1.35. The discrepancy in both cases is accounted for by the same value of condenser resistance. Observed resistance of a given loop is shown to vary greatly as condenser resistance is changed. Curves are run at 1.5×${10}^7$ cycles on No. 20 bright copper wire, No. 20 oxide coated copper wire, and No. 20 silver wire. For all curves the ratio fell well below 1.45 and was still decreasing as far as data were taken. Since curves run on bright copper wire coincide with curves run on exactly the same wire after it had gained a heavy coating of oxide, it can be definitely stated that the presence of oxide has no appreciable effect on the resistance.