The rate of recombination of ions in the copper arc is studied by means of the probe, the post-arc ion density being obtained directly as a function of time. Experimental results agreeing with the theory are obtained, and a coefficient of recombination, α = 5·7 × 10−9, is found. If the temperature of the gas in the path of the arc immediately after extinction is 2500°C., a comparison between the above value and that accepted for normal conditions shows that α varies approximately as the 2·5 power of the temperature, which is in agreement with the existing data.