Dwemer cogs are used in the Arniel's Endeavor questline, where it is referred to as a dwarven cog. Ten of them must be collected. Arniel Gane is secretive of their purpose in his research and requests that the Dragonborn mention them to no one.
Cogs may be found in numerous Dwemer Ruins:
- Several Dwemer cogs can be found in Mzulft during the quest for the The Staff of Magnus, so it is best to activate this quest with Arniel before setting out to Mzulft. Many of the cogs are hidden behind doors with expert- and master-level locks. In the Mzulft Aedrome area, there is a chest that may contain 5+ cogs in the last room down the hallway on the opposite side of the locked door. Three more can be found on the last floor of the Mzulft Observatory (just next to the buttons you must press to focus the Oculory) lying on the table with a few tomes of magic.
- South of where you arrive when fast-traveling to Mzulft, you can find four cogs located in the Dwarven Storeroom, along with several Dwarven Metal Ingots (must pick expert locks).
- Several can be found in Shimmermist Cave during the quest Trouble in Skyrim, which is given by Farkas in Whiterun.
- Mzinchaleft (southwest of Dawnstar) contains many cogs. Four of them are just behind the entrance gate to the dungeon, and two more are behind the locked door in the room with Maluril's Journal.
- The later Thieves Guild quests go through Dwemer ruins where several cogs can be found.
- At least one cog can be obtained from Dwarven spider workers in Avanchnzel during the Unfathomable Depths quest. There should also be three cogs sitting on a table in the Animoncultory, in the hallway right before reaching the boilery. Additionally, one cog is laying in the open on the far wall of the large square next to the stairs leading up to the boilery. Numerous others may be found in the disassembled Dwemer spiders and spheres lying on the shelves. These may be random, but should be common enough to find enough to complete the quest (it's possible to find 12 without significant exploring).
- Dwemer cogs can be stolen from the Dwemer Museum in Understone Keep, after Nimhe has been defeated for Calcelmo.
- One can sometimes be found in the Thieves Guild armory in the master lock chest.
- Two kids walking between Left Hand Mine and Hag Rock Redoubt sell Dwemer parts and also mark where they were found on your map.
- One Dwemer cog is located in Fort Dawnguard on the shelf next to the forge.
- Can find 10+ in Raldbthar which is east of Fellglow Keep in the mountains
- Can find several throughout Alftand.