In 2 1/2 qt saucepan combine brown sugar, corn syrup, butter, cream of tartar, and salt. Bring to a boil, stirring, over med high heat. Stirring constantly, boil rapidly to hard ball stage, 260F (about 5 mins). Remove from heat. Stir in baking soda quickly but thoroughly, pour at once over popcorn in a lg roasting or baking pan. Stir gently until all kernels are coated. Bake 200F for 1 hr, stirring 2-3 times during baking. Turn out at once on wax paper, spread apart and allow to cool completely. Break apart, store in a tightly covered container.