Preheat oven to 350 degrees F. Beat egg whites and cream of tartar in large bowl with electric mixer at medium high speed until soft peaks form. Combine sugar and salt in small bowl, gradually add to egg whites, beating at high speed until soft peaks form. Combine sweet potatoes, ginger, butter and orange juice in a medium bowl: spread in 2 quart casserole dish. Spread meringue over top of sweet potatoes. Bake about 14 or 15 minutes, or until meringue is golden. Serve immediately.