Petty Officer 3rd Class Nick Bean, Electrician's Mate at Coast Guard Aids to Navigation Team San Francisco, measures acid density for the backup battery at Alcatraz Lighthouse in San Francisco using a refractometer, Thursday, Jan. 14, 2016. ANT San Francisco members are charged with maintaining the structure and the navigational light for the lighthouse. (Coast Guard photo by Petty Officer 3rd Class Loumania Stewart)

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Keywords: 11, acid, aids, alcatraz, ant, battery, coast, densitiy, district, francisco, guard, island, lighthouse, navigation, refractometer, san