Preheat over to 200F Divide the popcorn between 2 large bowls. Oil 2 lipped cookie sheets very well and set aside. In a deep saucepan, combine the brown sugar, corn syrup, butter, salt and cream of tartar.
Insert candy thermometer. Bring to a boil and cook the mixture over medium heat for about 5 min, stirring occasionally, until the therm. registers 260F. (hard ball stage).
Remove from the heat and quickly stir in te baking soda. The mixture foams up. Pour the syrup mixture over the opocorn, half into each bowl. Work quckly to coat all the popcorn with the caramel.
Spread popcorn evenly on the cookie sheets and bake for 1 hour, stirring 3 times. Remove from the oven and spread popcorn on wax paper to cool, breaking up the larger pieces, if necessary. Store in airtight container.
Storing it would never be a problem in my house, eat it up to fast!!!