Unlike cyclones these are used to collect very small particles. A common configuration consists of vertical wires are placed between parallel collector plates in a "flow cell". A strong field is created when a high negative voltage (100000V) is applied to the wires. The plates are earthed and a corona discharge is formed. (Refer to an 'A' level Physics TextBook if you do not understand).

The discharge ionises gas molecules in air stream. Negative ions interact with particles, which pick up a negative charge and are attracted to plates. They discharge on contact with the plates.