Theodore Lyman

Zoologist and soldier, born in Waltham, Massachusetts, USA. He studied at Harvard (1855) and worked under Louis Agassiz. He was one of the first trustees of the Museum of Comparative Zoology, and he pursued his studies both at home and abroad. He volunteered during the Civil War and was present at several important occasions. The letters he wrote to his wife, published as Meade's Headquarters, 1863–65, Letters of Col Theodore Lyman from the Wilderness to Appomattox (1922), are a valuable source for students of the Civil War. He was a member of the House of Representatives (Independent, Massachusetts, 1883–5).

He became an assistant in physics at Harvard, where he remained, becoming full professor in 1917, and where he was also director of the Jefferson Physical Laboratory (1908-17). Dr. Lyman made important studies in phenomena connected with diffraction gratings, on the wave lengths of extreme ultraviolet light discovered by Schumann and also on the properties of light of extremely short wave length, on all of which he contributed valuable papers to the literature of physics in the proceedings of scientific societies. The Lyman Crater, an impact crater that lies in the southern hemisphere on the far side of the Moon, was named after him.

Directivity is also defined for directional couplers and similiar devices for RF and Microwave measurements. It is the properof the device couple power in one direction while rejecting in other direction. For example in a directional coupler if the directivity of a measuring port is 40 dB, it means the power coupled in this port from desired direction will be 40 dB higher than the undesired power coupled from other direction.