Following graduation Forrest took up his first appointment, as a physicist with the
Central Electricity Board, Glasgow, followed the next year (1931) with that of physicist with the Central Electricity Board at London, working on the new
National Electricity Grid. His first published paper on the Grid was submitted to the
Institution of Electrical Engineers in June 1931, in which month he also attended the Conference Internationale des Grands Reseau Electrique at Paris, with
Thomas Allibone. Forrest's third Paper,
The electrical characteristics of 132 kV line insulators under various weather conditions was submitted to the
IEE in March 1935, and a further Paper, written in 1940, was sent in February 1941, winning the Coopers Hill War Memorial Prize. In 1942 Forrest was elected a Fellow of the
Royal Meteorological Society, later becoming one of the founder-editors of the publication,
Weather, one of his own Papers appearing in the publication's first volume, 1946. Forrest organized a joint meeting, October 1945, of the IEE and the
RMS on the effects of weather on power systems. He continued to write scientific Papers and in 1948 was awarded
D.Sc. by Glasgow University. Rising to Directorship of the new Central Electricity Research Laboratory with some 800 staff, in 1960 Forrest was invited to give the Hunter Memorial Lecture and was nominated Chairman of the Supply Section (1961–62) of the IEE, and in 1963 he gave the third
John Logie Baird Memorial Lecture in the
Royal College of Science and Technology and was elected President, Section A of the
British Association for the Advancement of Science in Aberdeen. From November 1963 through to April 1964, Forrest gave a series of thirteen deliveries of the
Faraday Lectures, plus repeats for sixth form school pupils, to a total audience of some 35,000. Also in 1964, Forrest was invited to become visiting professor at the new
University of Strathclyde. By 1966, Forrest had become a member of three of the study committees of the Conference Internationale des Grands Reseau Electrique (and became Chairman of the British National Committee in 1972) and in that year, as President of Section A of the British Association for the Advancement of Science, he was invited to attend the meeting of the Indian Association (which became the
Indian Science Congress) meeting in Hyderabad in 1967 where he delivered a lecture on the British electricity supply industry. In 1966, Forrest was elected a Fellow of the
Royal Society and in 1972 elected to its Council. Also in 1972, he was awarded an Hon.
D.Sc. by
Heriot-Watt University, Edinburgh. ==Post retirement==