Experimental work was performed to determine the relative amount of interference caused by discharges on line insulators due to their configuration, to the atmospheric conditions, and to pollution of the insulator surfaces. Control of conditions such as humidity was obtained by investigations within a glass-sided chamber. The cap-and-pin type insulator was most free from discharges, but methods of reducing the discharges considerably were found for the pin-type insulator by means of metal insets to the binder or by the use of metal caps; and for the interlink-type insulator by using large links and compound lining. Field tests were made ot verify the conclusions reached from the laboratory work. Several points of agreement were noted.