Dynamically increasing textbox height using jquery

<script type="text/javascript" src="http://code.jquery.com/jquery-latest.min.js">
</script>

<textarea id="textbox" style="height:20px;width:400px;"></textarea>

<script type="text/javascript">
$(document).ready(function() {
    $("#textbox").val('');
    $("#textbox").keypress(function() {
        var textLength = $("#textbox").val().length;
        if (textLength % 50 == 0) {
            var height = textLength/50;
            $("#textbox").css('height', 20+(height*20));
        }
    });
});
</script>