In the beginning, the Cottrell Process of Electrical Precipitation was greatly handicapped by the fact that there did not exist standard electrical equipment which could be used to develop the necessary potentials under the severe conditions imposed by the mechanical rectifier, used for rectifying the high-potential alternating current. The transformers used were a constant source of trouble and annoyance. This condition delayed the rapid accumulation of accurate data pertaining to precipitattion phenomena, since those engaged in the work were kept busy in merely maintaining a source of power. It was not long, however, before the electrical manufacturing companies were interested in the problems involved, and better transformers were produced. These transformers have now been developed to a degree which is very nearly the equal of the ordinary power transformiers of the same voltage ratings. For several years there was a demand for higher and higher voltage ratings, owing to the belief that through the use of very high potentials and consequent large electrode spacing the size and cost of precipitators could be greatly reduced. Potentials as high as 250,000 volts were experimented with, but such high voltages proved to be impractical. Transformers as now standardized are in two voltage ranges, one with a maximum voltage of 75,000 volts having taps in the low tension to deliver 50, 60, 65 and 70 kilovolts, the other having a maxim nam voltage rating of 100,000 volts with taps in the low tension to deliver 50, 62?, 75 and 87? kilovolts.