The Anti-Mind Reading Helmet is one of Mr. Crocker's fairy detecting/capturing devices. It only appeared in the episode Mind Over Magic.
The machine is a bowl-shaped helmet that is worn on the head and weighs a ton. It requires support just so Mr. Crocker can stand with it on, and whenever he spasms about fairies, he usually breaks the sticks and is promptly crushed by the device. He solved this by putting it on a pulley system when he used in the gym at the school.
Though the device's weight made it extremely inconvenient, it performed its intended function perfectly; when Timmy attempted to read Crocker's mind while the latter was wearing the helmet, the power bounced off, causing Timmy to read his own mind instead.