Refer to figure.
SMITHS ICE DETECTOR
The Smiths ice detector (also known as differential pressure ice accretion detector) consists of a hollow tube which contains a series of small holes and one large hole positioned in its leading edge and a separate series in its trailing edge. The detector monitors pressure differential between the two edges. In flight under normal conditions, the holes remain open and, consequently, the pressure differential is zero. In icing conditions, holes in the leading edge rapidly become blocked by ice. This causes a change in the pressure differential - a decrease in dynamic pressure is sensed, causing the relay unit to give a warning. A heater element is fitted around the tube to dissipate accumulated ice, which heats up the small holes first causing the indications to turn off.