Jq如何获取并操作iframe中的元素？

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
    <title></title>
    <script src="Scripts/jquery-2.2.2.min.js"></script>
</head>
<body>
    <iframe id="frm1" allowtransparency="true" frameborder="0" width="560" scrolling="no" src="/demo.html"></iframe>
    <script>
        //jq给iframe中的文本框赋值
        $("#frm1").load(function () {
            var frm = $("#frm1");
            frm.contents().find("").val("给元素赋值");
        });
         
    </script>
</body>
</html>