An instrument has been designed for measuring the dielectric constant of a material in the form of a single cylindrical filament, and experiments are described in which it is used for measurements on nylon. The filament is placed centrally between two silvered optical flats, which form a parallel-plate capacitor and serve as the tuning capacitor of a radio-frequency oscillator. According to an existing theory, the change in capacity due to the presence of the dielectric is given by:δC = ((b²H/2D²) tanh πB/2D)/((ε + 1)/(ε - 1) - π²b²/3D²)(where b is the radius of the filament, H the length of the capacitor, B the width and D the distance between the plates) and δC can be determined from the corresponding change in oscillator frequency, if the electrical parameters of the circuit are known. The experimental observations confirmed the general form of the equation, but they were not sufficiently accurate to verify the second-order term π²b²/3D². A value of 3.0 was found for the dielectric constant of nylon at 6 to 7 Mc/s, which is in reasonable agreement with values of 3.1 to 3.4 recorded by other workers for both filament and bulk nylon.