Taking the entire text

I m hvng a text field and an observer attached to that field.
<%= text_field_tag :my_id%>
<%= observe_field (:my_id,
:url=>{:action=>‘do_this_when_changed’}) %>
The string textfield can be very long. Thus observer is taking only the
part that is visible at one go and not the entire string… How can i
make it take the full string??? Plz help.

Thnx and regards,