请教：计时器无法停止，代码哪里出错了呢?
<script type="text/javascript">
function clock(){
var time=new Date();
document.getElementById("clock").value = time;
}
var i=setInterval("clock",100);
</script>
</head>
<body>
<form>
<input type="text" id="clock" size="50" />
<input type="button" value="Stop" onclick="clearInterval(i)" />
</form>
</body>
