Barcelona (Freddie Mercury and Montserrat Caballé song) contained "Exercises in Free Love" from "The Freddie Mercury Album". The 1992 version of the 7" Spanish record had another version of "Barcelona" on the B-side, and the rare 1987 12" Hong Kong record was one-sided and had a unique sleeve. Most 5" CDs contained two or three versions of "Barcelona" and "Exercises in Free Love". Most 7" record and 5" CD covers featured Mercury and Caballé singing on a stage with an orchestra on the background, though the Portuguese version pictured them in a static studio photo. The 1992 reissue 7" records contained another version of the singing artists,
Mercury battery popular power source for portable electronic devices. Due to the content of toxic mercury and environmental concerns about its disposal, the sale of mercury batteries is now banned in many countries. Both ANSI and IEC have withdrawn their standards for mercury batteries. The mercury oxide-zinc battery system was known more than 100 years ago but did not become widely used until 1942, when Samuel Ruben developed a balanced mercury cell which was useful for military applications such as metal detectors, munitions, and walkie-talkies. The battery system had the advantages of long shelf life (to 10 years) and steady voltage output.