When an em (electromagnetic) wave of fixed frequency is incident on the cylindrical positive column of a low‐pressure mercury vapor arc discharge with its electric field and direction of propagation both perpendicular to the column axis, a spectrum of resonances of reflection and transmission occurs as the discharge current of the positive column is varied. In addition to the expected cylindrical plasma resonance, a set of lesser resonances may be distinguished. The locations of these resonances have been extensively studied experimentally as functions of temperature and signal frequency for four tubes of different cross sections. It is shown that temperature, which is governed by losses in the plasma column and by the ambient temperature, has a substantial effect on the location of the resonances. By using this effect, spectra of resonances are easily obtained over a frequency range of 4400 to 22 000 Mc for the smallest tube. In addition, the sharpness of the cylindrical plasma resonance is calculated from experimental data and compared with theoretical curves ofQwhich have been determined from equations which take into account radiation and collision damping.