THE determination of the velocity of light has attracted the attention of many scientific workers and, in spite of the experimental difficulties involved, its value is now known with a probable error of about 5 parts in 105. The velocity of electromagnetic waves, represented in Maxwell's theory by the quantity where µ is the permeability and k the permittivity of the medium, is generally assumed to have the same value as that of light; and although the electrical measurements made so far are less accurate than those of the velocity of light, they have served to establish the equality of the two constants within the limits of experimental error. Other electrical methods have become practicable as a result of the techniques employing centimetric waves developed during the last few years. The one dealt with in this note depends on the electrical resonance of a short length of a wave-guide closed at both ends. The frequency of resonance of a cylindrical guide of diameter D and length L is in which v represents r is a constant for a particular mode of resonance and n is the number of half wave-lengths in the guide.