getElementByID when script is external

I am trying to access a asp:panel in an external script to make it not visible,but it does not seem to be working. When the Script is in the .aspx file it works fine though.Any suggestions?

In the .aspx file

  <script src="App_Themes/custom.js"></script>
  <asp:Button ID="descriptionButton" Text="Description" runat="server" OnClientClick="descButton(); return false;" />

 <asp:Panel ID="desciptionPanel" runat="server"> 
          ///random stuff
               <asp:panel>

in the custom.js file

function descButton() {

var desc = document.getElementById('<%=desciptionPanel.ClientID%>'); 
desc.style.visibility = "visible";
desc.style.height = "800px";

}

Thanks in advance