polarity of the neutron flow" twice on screen – in The Sea Devils (1972) and the 20th Anniversary special The Five Doctors (1983). Pertwee used the phrase when he acted in the stage play Doctor Who – The Ultimate Adventure in 1989. When Colin Baker took over the role in the play he amended the line to "Reverse the linearity of the proton flow." In the radio play The Paradise of Death, the Brigadier asks "Reverse the polarity of the neutron flow?" and the Doctor proceeds to explain that the phrase is meaningless (though in reality neutrons can be polarized by a magnetic field, such that reversing the magnetic field’s direction reverses the polarity of the neutrons). On other occasions on screen, the Third Doctor "reversed the polarity" but not of neutrons. The full phrase was used in several Target novelisations. It was subsequently used by the Fourth Doctor (in City of Death) and the Fifth Doctor (in Castrovalva and Mawdryn Undead). Together with The Five Doctors, this resulted in the phrase being used as a nostalgic reference. In the Tenth Doctor episode "The Lazarus Experiment", the Doctor, while hiding in Lazarus' machine, comments that it had taken him too long to reverse the polarity due to being out of practice. The Tenth Doctor uses the full phrase in "Music of the Spheres". During the episode "The Almost People", a clone of the Eleventh Doctor speaks the phrase while reliving the memories of all his predecessors. He goes on to conflate it with his regeneration-spanning love of jelly babies, remarking that they need to "reverse the jelly baby of the neutron flow". In "The Day of the Doctor", the Eleventh Doctor invokes the phrase when confronting a time portal with the Tenth Doctor, suggesting that they both "reverse the polarity" with their sonic screwdrivers (which merely neutralizes each other’s efforts). In "The Girl Who Died", the Twelfth Doctor tells Clara Oswald he is "Reversing the polarity of the neutron flow", followed by "I bet that means something. It sounds great." Clara herself uses the phrase, saying she "reversed the polarity" of a mind-wiping device to prevent the Doctor from erasing her memories of him from her mind ("Hell Bent"). In "It Takes You Away", Yasmin Khan suggests that the Thirteenth Doctor reverse the polarity on the sonic screwdriver in order to (successfully) open a locked inter-universe portal. Title sequence and