rails和ajax请求

I have started to learn rails and javascript. How i canmake properly ajax request to rails controller with jquery? I write in js file

$.get('http://localhost:3000/take/show',{},function(s){
           alert(s);
          },'text');

in controller:

class FirstController < ApplicationController
  def show 
    render :json => "Hi!"
  end
end

but instead i see only blank alert dialog. What i have done wrong? In all tutorials i see thatthe URL in $.get should be like this "take/show" but it would not work in my case, so why?