Get radio value inside iframe

<form id="myForm">
  <iframe>
    <!-- Normal HTML headers omitted -->
    <input type=radio name="myRadio" value=1>First
    <input type=radio name="myRadio" value=2>Second
    <input type=radio name="myRadio" value=3>Third
  </iframe>
  <input type=button value="Submit" />
</form>

$("input[@type=radio][@checked]");

var frameDocument = $('#myForm iframe').contentDocument || $('#myForm iframe').contentWindow.document;
$(frameDocument).find('input[type=radio][checked]');