How to apply preloader on .load(url) function

Question

I want to apply preloader div on <select>. when i select any option from the dropdown, it is showing relevant topics from other page in my domain by using appended jquery.

$("#myselect").change(function(){
$(".showpost").load((mysite_url_to_post) + " " +".posts_container", function() {
  });
});

i want to apply preloader when we select any other option from dropdown. my issue is that my preloader is not working on changing the option. any help is highly appreciated. Here is how i am trying to achieve this: my html:

<div class="showpost">
<div class="main_loader">
<div class="my_loader">
</div></div></div>

my query for preloader:

    $('.main_loader').fadeOut(500);