Essen (1950). Essen, Louis (1950), »The Velocity of Propagation of Electromagnetic Waves Derived from the Resonant Frequencies of a Cylindrical Cavity Resonator«, Proceedings of the Royal Society of London A, 204 (1077): 260–277, Bibcode:1950RSPSA.204..260E, doi:10.1098/rspa.1950.0172, JSTOR98433
Essen (1950). Essen, Louis (1950), »The Velocity of Propagation of Electromagnetic Waves Derived from the Resonant Frequencies of a Cylindrical Cavity Resonator«, Proceedings of the Royal Society of London A, 204 (1077): 260–277, Bibcode:1950RSPSA.204..260E, doi:10.1098/rspa.1950.0172, JSTOR98433
Essen (1950). Essen, Louis (1950), »The Velocity of Propagation of Electromagnetic Waves Derived from the Resonant Frequencies of a Cylindrical Cavity Resonator«, Proceedings of the Royal Society of London A, 204 (1077): 260–277, Bibcode:1950RSPSA.204..260E, doi:10.1098/rspa.1950.0172, JSTOR98433