Kirk's new fashion statement
A polarized conductor worn as a headband
A polarized conductor was a type of conductor that was devised by the women who inhabited Planet Two the Taurean system and was worn as a headband.
When the women were near an individual, the headbands worn by the men glowed brightly; when they are not near, the glow diminished. Spock postulated that the headbands transferred the vital energy or life force from humanoid males to the bodies of the females. Prior to receiving this life energy, the women appeared listless. However, the headbands also caused the males affected to undergo rapid aging and weakness, which in turn made the women more energetic and vital. The aging process caused by the headbands caused men to age at a rate of ten years per day.
In 2269, the crew of the USS Enterprise visited the Taurean system in an attempt to solve a mystery concerning lost crews and ships. The male crew members, including Kirk, Spock, McCoy, and Carver, almost succumbed to the life-draining devices, but were saved by the female officers, led by Uhura and Christine Chapel. (TAS: "The Lorelei Signal")
Differing from the animated series, in the novelization of "The Lorelei Signal" (published in Star Trek Log 2), the headbands are described as consisting of "a gold headband with a large blue gem."