method of removing certain impurities (either solid particles or liquid droplets) from air or other gas. It is widely used for removing particles from industrial-waste gases.

In 1824 M. Hohlfeld, a mathematics teacher in Leipzig, Ger., first described the precipitation of smoke particles by electricity. The first commercially successful process was developed in 1906 following experiments by F.G. Cottrell at the University of California, Berkeley.

When a discharge of electricity is fed into the air or gas, the particles it contains are ionized; i.e., they become negatively charged from the gain of electrons on their surfaces. A large electrode surface of opposite charge then attracts these particles, and they are deposited there for removal by washing or scraping.

The earliest applications were to the smelting and sulfuric acid industries and at cement mills. Other important applications include removal of tar from coke-oven gases; cleaning the air in air-conditioning systems; removal of acid fumes in petroleum refineries and chemical plants; recovery of such industrially valuable materials as the oxides of tin and copper; and removal of fly ash from stack gases of large coal-burning power plants.