The Anti-morphing ray (often abbreviated as 'AMR') was a Yeerk invention created to force an Andalite (or human, as the case may be) to demorph. Only one prototype AMR was ever created. It was first mentioned in The Separation but wasn't featured until The Illusion where the Animorphs fooled the Yeerks into believing it didn't work. They accomplished this by Tobias morphing into Ax and pretending to be an Andalite morphed into the body of a hawk. The AMR is described as looking 'remarkably like the kind of telescope an amateur astronomer might own'. Visser Three is convinced the AMR is a failure, and has its inventors fed to the Taxxons. What happened to the AMR after that is never revealed, but it was most likely destroyed in the battle between the Animorphs and Taylor's forces
Whether or not the AMR would have worked is never revealed. Tobias describes feeling an 'ever-so-slight tingle' when he is targeted by the AMR, but as he is not morphed, the device has no effect on him.
Taylor uses a nervous system manipulation device to torture Tobias in The Illusion that uses the exact same three-button user interface as the AMR.