In a stand mixer, beat together the egg whites, cream of tartar and salt on high speed until soft peaks form.
Meanwhile, boil together sugar and water in saucepan until it reaches thread stage (about 230 degrees F to 234 degrees F on candy thermometer), about 7 minutes.
While beating egg whites at medium speed, slowly dribble in hot syrup. Increase speed to medium-high and continue beating 5 minutes, until meringue is cool and forms shiny, firm-soft peaks that hold their shape. Makes about 10 cups.