- “Ford got out his Sub-Etha Sens-O-Matic. It was making vague humming noises and a tiny light on it was flickering faintly. "Flat battery?" said Arthur.”
- —From Life, the Universe and Everything[src]
The Electronic Thumb or Electronic Sub-Etha Signaling Device was a device which allowed hitchhikers to flag down passing spacecraft to hitch a lift. Ford Prefect owned one of these and used it to get himself and Arthur Dent off Earth, moments before it's destruction, and onto a nearby Vogon spaceship.
Successfully attracting a passing spacecraft depended on the proximity, sensitivity and active mode of the spacecraft's receiver, and the desire and willingness of the craft's occupants to put up with a hitchhiker for the next leg of their journey.
The thumb was originally described as a short squat black rod, smooth and matte with a couple of flat switches and dials at one end. The signal was said to be akin to an emergency signal in that it was omnidirectional and limited in range.
In the film adaption, it appeared as a smooth, silver ring, with a symbol on it that lit up brightly when it was used.
In all three of the original dramatic adaptations, Ford owns an electronic thumb as part of his role as a researcher for the guide and uses it to hitchhike across the galaxy. It is a black rod with switches on one end, used to signal passing spaceships. At the start of the story, Ford successfully hitches a ride onto a Vogon ship from the Vogon Contructor Fleet, with Arthur in tow, due to the Dentrassi cooks allowing them on board.
Arthur Dent finds the electronic thumb in Ford's bag just before they are taken aboard the Vogon spaceships. It has two controls on it, both of which are needed to advance the plot: one which signals passing spaceships, the other which contacts the engineering firm that built the electronic thumb to dispatch a field engineer to help in repair work.
In the first case, Arthur (through the player) has only a few turns to figure out to push the first button in order to be saved by the Vogons. In the second case, Arthur can also use to escape from the belly of a sperm whale, when pressing the first button does not help him escape. Pressing the other button summons an engineer robot to repair it.
In the film, the electronic thumb is a metallic ring that is worn on - obviously - the thumb. Holding out one's thumb sends a gleaming beacon light out into the sky, summoning passing spaceships. Ford uses it to help himself and Arthur escape from the Earth just before the Vogons destroy it. Almost as soon as they get aboard the ship, Ford tries sending the signal out through the piping to get off that ship, but cannot get a signal out, giving the Vogon Guards time to find them and throw them out into space.
- In the radio and television series and the video game is called the "Electronic Sub-Etha Signaling Device," not to be confused with the Sub-Etha Sens-O-Matic.