THE Duddell Medal for 1930 of the Physical Society was presented to Sir Ambrose Fleming at the annual general meeting of the Society on Mar. 20. The medal is awarded not more frequently than once a year to persons who have contributed to the advancement of knowledge by the invention or design of scientific instruments or by the discovery of materials used in their construction. Sir Ambrose Fleming's connexion with the Physical Society dates back to its very beginning, for he read the first paper at the inaugural meeting of the Society in March 1874. In 1879, Sir Ambrose designed a special form of resistance balance for comparing standard coils, and a special form of standard coil capable of taking up more quickly the temperature of the water in which it was placed than the form before in use. When practical incandescent electric lighting began, and quick and accurate workshop methods of electrical measurement became necessary, he made, in 1883, the first direct reading potentiometer set to read directly current and potential in amperes and volts by means of a standard Clark or Weston cell. Sir Ambrose also designed a form of wattmeter, with which he made extensive researches on alternating current transformer efficiencies, reported to the Institution of Electrical Engineers in 1892. In this paper he first suggested the use of the term ‘power factor’, which at once came into everyday use.