Clear reception of radio signals depends upon the ability to exclude unwanted noise. The chief cause of man-made noise is electrical circuit transients which cause radiation from portions of circuits where sparks occur. Typical causes are: switches, motors, ignition systems, X-ray apparatus, diathermy machines, industrial precipitators, high-frequency heating appliances, domestic appliances, etc. The problem may be aggravated by the use of a large number of such devices in the same area.