Dwemer Cog

Dwemer Cogs are used in Arniel's Endevor 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.

Weight
Dwemer Cogs at first seem to be unusually heavy, but after examining a Dwarven Sword which weighs 12 units, it seems much more realistic.

Locations
There are cogs in numerous Dwemer Ruins:
 * Several Dwemer Cogs can be found in Mzulft, during the quest for 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 Staff of Magnus, Mzulft Aedrome area, in the last room in a hallway on the opposite side of the locked door, the chest may contain 5+ cogs. 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.
 * There are also four located in the Dwarven Storeroom south of where you arrive when fast traveling to Mzulft, along with several Dwarven Metal Ingots (must pick expert locks).
 * Several can be found in Shimmermist Cave during the "Trouble in Skyrim" quest given by Farkas in Whiterun.
 * Mzinchaleft (southwest of Dawnstar ) contains many cogs. Four of them are just behind the entrance gate to the dungeon, two more are behind locked door in room with Maluril's Journal.
 * The later Thieves Guild quests go through Dwemer ruins where several can be found.
 * At least one of them can be obtained from Dwarven Spider Workers in Avanchnzel, during the Unfathomable Depths quest. There should also be 3 cogs sitting on a table in the Animoncultory, in the hallway right before reaching the boilery. Additionally there shoud be one laying in the open on the far wall of the large square next by the stairs leading forward to the boilery. Numerous others may be found in the disassembeled 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.
 * 2 kids walking between Left Hand Mine and Hag Rock Redoubt sell Dwemer parts and also mark where they were found on your map.