After defining the skin-effect resistance and reactance ratios, the "spirality" and "proximity effects" for conductors at radio-frequencies, a differential bridge circuit supplied with current from 10,000 to 100,000 cycles by an Alexanderson alternator is described. There are tested at various frequencies straight solid wires, stranded wires (of various spiralities, and also braided), wires with definitely spaced strands, very finely stranded wire (with insulated strands), braided "litzendraht" wire, stranded twisted wire (of various spiralities), strips (both singly and in opposed proximity), and tubes. Important practical design data, quantitative as well as comparative, are given for the range of frequencies investigated. In a number of appendices to the paper, the details of the apparatus and certain theoretical calculations of radio-frequency resistance are given.