A counter has been used for the estimation of the mobility spectrum of positive large ions in the atmosphere of a laboratory containing high-voltage equipment producing positive corona. The form of the mobility spectrum is changed relative to that under natural conditions. The densities of ions of all mobilities, of the same polarity as the equipment, are increased. In conjunction with a small-ion counter, measurements have shown a saturation in the total density of large and intermediate ion mobilities, as a function of increasing corona voltage, in the presence of an increasing density of small ions. Measurements of the decay and recovery of ions of, respectively, the same and opposite polarity to the corona voltage, have shown that after corona ceases, densities of ions of opposite polarity which are initially very low, are very slow to recover, especially in the case of small ions. It is concluded that atmospheric ionisation conditions are disturbed for at least 1 h after operation of high-voltage equipment. Parallel experiments in the presence of negative corona show similar trends.