Mesmetron

Mesmetron is a Fallout 3 weapon that is used in the quest Strictly Business, and is given to you by Grouse (the Paradise Falls entrance guard) when you first begin the quest. When used on the specified quest NPCs, it puts them into a hypnotic stupor, allowing the player to rob and enslave them (by putting a Slave Collar on them) and send them to Paradise Falls. It also appears to work on some other NPCs, but often simply sends them into a berserk frenzy in which they become hostile towards the player and every other NPC in the area. Occasionally, the victim will run several steps before their head explodes.

It uses Mesmetron power cell as ammo. You start out with 50 Mesmetron power cells. Grouse will also sell you additional power cells, but at a fairly steep price (200 caps for 10).

If you don't want to take the Strictly Business quest, you can obtain the Mesmetron by killing Grouse and looting his corpse. You have to kill him to obtain the Mesmetron, as it cannot be pickpocketed. If you kill Grouse, the Mesmetron can only be used for robbing or frenzying NPCs, as you cannot enslave NPCs with Grouse dead and Paradise Falls hostile towards you. You also cannot obtain additional Mesmetron power cells from anyone other than Grouse.

Background
The Mesmetron is an experimental pre-war "non-lethal pacification" weapon that was being tested in the months immediately preceding the nuclear war. It operates by sending a signal pulse into a human target's brain that scrambles higher-level mental functions, leaving the target extremely susceptible to suggestion. Still in its testing phase, the weapon was found to have the potential to trigger two undesirable side-effects... 1) extremely increased aggression, or 2) "spontaneous cranial eruption".

Grouse found it in a pre-war military installation, and gives it to the player character for field testing if they accept the Strictly Business enslavement quest from him.

Trivia

 * The Mesmetron is similar in both name and effect to the Persuadertron from the Syndicate game series. The Persuadertron, however, allows complete domination over a victim without the need of a Slave Collar.
 * The idea of an exploding Slave Collar is featured in quite a few movies and novels, such as:
 * The Running Man where people are held captive through the use of collars around their necks that can be triggered to explode.
 * Another film making use of an "exploding collar" to make prisoners obey is Wedlock . Also widely known as Deadlock.
 * Exploding collars are also used in the movies Battle Royale and Battle Royale 2: Requiem, and in the 3-part anime series Cyber City: OEDO 808.
 * The Playstation 2 game Ratchet: Deadlocked/Ratchet: Gladiator (outside the US) features exploding slave collars. In co-op, these explode when the two players are too far away from each other for ten seconds.
 * An exploding collar was prominent in the unusual death of Brian Douglas Wells, also known as The Pizza Bomber.
 * In Episode 2 of Strong Bad's Cool Game For Attractive People, Strong Bad is placed under house arrest using an exploding collar when he refuses to pay an arbitrary email tax.
 * Exploding collars also appear in the movie "Battlefield Earth'
 * In an Episode of Invader Zim, it is shown that the School Hall Passes are Collars strapped around the neck that explode if a student wearing one were to step off school grounds. It is also shown that when the collar is in use by another student, there is another hall pass, called the "Auxiliary Hall Pass", which is a heavy old green radiator heater with Hall Pass spray painted in red on the side, which has to be pulled around by the student.
 * Similar to the L.O.O.K.E.R. (Light Ocular-Oriented Kinetic Emotive Responses) gun, a light pulse device that gives the illusion of invisibility by instantly mesmerizing its victims into losing all sense of time.
 * "The device that you're wearing is linked to my heart rate monitor. The second that heart rate monitor flatlines or you move out of range, an explosion will go off in that collar. Your life and my life will end simultaneously." A quote from the horror movie sequel Saw 3 from the evil mastermind himself, Jigsaw.
 * Similar device is used by Max Eilerson (archeologist from Babylon 5:Crusade) on thug leader to force him to stop bothering Max's family.
 * In the Japanese anime One Piece, slaves wear exploding collars that can be triggered by their owners whenever necessary.
 * In the Warhammer 40,000 universe, the non-playable faction the Adeptus Arbites, essentially the police force of the Imperium, is mentioned to use prisoners with exploding collars as troops.

Appearances
Mesmetron appears only in Fallout 3.