var http_request_null = false;
  function fnRequestNull() {
    http_request_null = false;
    if (window.XMLHttpRequest) {
      http_request_null = new XMLHttpRequest();
      if (http_request_null.overrideMimeType) {
        http_request_null.overrideMimeType("text/html");
      }
    } else if (window.ActiveXObject) {
      try {
        http_request_null = new ActiveXObject("Msxml2.XMLHTTP");
      } catch (e) {
        try {
          http_request_null = new ActiveXObject("Microsoft.XMLHTTP");
        } catch (e) {}
      }
    }
    if (!http_request_null) {
      return false;
    }

    http_request_null.open("GET", "null.htm", true);
    http_request_null.send(null);
  }