Complex sentence: He then worked for the National Institute of Industrial Psychology between 1930 and 1935, becoming a consultant to JS Rowntree & Sons, where he was intimately involved in the design and marketing of Black Magic chocolates.
Split into simpler sentences: He then worked for the National Institute of Industrial Psychology between 1930 and 1935. For part of this time he was a consultant to JS Rowntree & Sons, where he was intimately involved in the design and marketing of Black Magic chocolates.