Agner Krarup Erlang, Danish mathematician, c 1915.
© Science Museum / Science & Society Picture Library
Etching of Erlang (1878-1929), pioneer in the study of telecommunications traffic. Erlang wrote a formula to calculate the proportion of callers to a village telephone exchange who would have to wait when trying to ring someone outside the village. His first work, 'The Theory of Probabilities and Telephone Conversations' (1909), brought Erlang worldwide recognition, and his formula was accepted for use by the British General Post Office. Erlang worked for the Copenhagen Telephone Company for nearly 20 years. During the 1940s the 'Erlang' became the accepted unit of telecommunication traffic measurement. It has been used ever since in the design of modern telecommunications networks.