[jQuery] Easy typewriter plugin, need some help with scope

[DanDan]

Here's my code.  The plugin should take the text from a container,
erase the container, then print out the text one character at a time
in a typewriter fashion.  The problem is that setInterval() seems to
think that addText() is undefined, because setInterval() runs at the
'window' scope level.  I had this function running fine as a normal
program, but I'm unclear on how to convert it over to the plugin
format and solve the scope issue.  thanks in advance.

jQuery.fn.typewriter = function(speed) {
        return this.each(function(){

                var contents = jQuery(this).html();
                var count = 1;

                if(contents){
                        jQuery(this).html("");

                        var interval = setInterval("addText()", speed);
                }

                function addText(){

                        jQuery(this).html(contents.substr(0, count));

                        count++;

                        if(count > contents.length){
                                clearInterval(interval);
                        }
                }

        });
};