The paper describes an attempt to determine experimentally the accuracy of Butterworth's formulae for the highfrequency resistance of single-layer solenoids. The investigation has been carried out on 4-in. diameter air-spaced coils of 10, 30, and 50 turns over the frequency range from 2 × 105to 2.025 × 106cycles per sec. The coils were wound with bare wire of diameter 1.63 mm. (No. 16 S.W.G.), with a distance of'2.60 mm. between the centres of adjacent wires. The method of measurement described was based on the use, for each coil shape, of a series of identical coils wound with copper, aluminium, brass, german silver, and eureka wire, respectively. There is some uncertainty in the interpretation of the results, but, in spite of this, it is evident that of the two formulae with which a comparison is made, one of them is correct to within about 4 per cent for the 30- and 50-turn coils over the whole frequency range considered. This formula is quite inapplicable to the 10-turn coils; however, for these the second formula gives reasonably good agreement with the experimental results.