3秒后自动跳转到另一个页面

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
</head>

<body>
<span >请等待！！</span> 
<span id="second" >3</span>  
  
<!--  跳转-->
   
  
  <script type="text/javascript">    
    var num = document.getElementById("second").innerHTML;  
   //获取显示秒数的元素，通过定时器来更改秒数。  
    
   function count()  
    {  
        num--;  
        document.getElementById("second").innerHTML=num;  
        if(num==0)  
        {  
            location.assign("tiaozhuan.php");  
       }  
   }  
    setInterval("count()",1000);  
  //通过window的location和history对象来控制网页的跳转。  
    function back()  
    {  
       window.history.back();  
  }  
     
 </script>   
</body>
</html>