Probe measurements have been made in BARTOL RESEARCH Fotmdation cadmium, thallium and carbon arcs by sweeping the probe Communication 1%. 2s. through the arc at a constant velocity and measuring the current collected by the throw on a ballistic galvanometer with different probe potentials applied. The voltage-current characteristic was analyzed to get a measure of the positive ion concentration, the average electron temperature and the space potential by the Langmuir method. The cathode falls found were each equal to, or only slightly higher than, the ionization potential of the active gas, and were 9.o volts in the cadmium arc, 6.5 volts in the thallium arc and 5.o volts in the carbon arc. The last value is to be compared with the ionization potential of the cyanogen molecule of 4.4 volts. The anode fall found in the carbon arc was I6.5 volts. These values for the carbon arc agree almost exactly with the "forward" and "back" E.M.F.s measured by Duddell and ot hers at the cathode and the anode but not identified by them as the cathode and anode falls. The electron velocities were found to be Maxwellian with average velocities sufficiently high to account for the ionization known to exist. Over parts of the arc stream where the fields were found to be large the electrons showed corresponding increases in average temperature and evidence that only about threequarters of the energy gained by the electrons from the field between collisions is lost by the lectrons while the remaining quarter is retained and made evident by the steady increase in the electron temperture. As the current flowing through the are was increased, the radiation per unit volume and also the positive ion current per unit length of the probe were found to increase very rapidly indicating an increase in the efficiency of the ionization process as the current increases. It is thought that this observation is closely related to the fact that the voltaKe drop across the arc decreases as the arc current increases.