Throughout the trilogy, we see Frodo carry the One Ring on a silver chain around his neck, rather than carrying it in his pocket as Bilbo does. The chain is given to him in Rivendell, it is made of elvish steel, and although the presentation of the elves differs from the books to the films, one thing which remains consistent is the power and quality of their craftsmanship. Subsequently, the gifted chain possesses some interesting qualities that help Frodo to resist the urge of the ring and its calling to its true master Sauron.
There are 3 main abilities/ gifts that the chain gives to the ring bearer to help him in his monumental task of taking it to be destroyed in the fires of Mount Doom where it was created. The first of which is the most subtle: skin-to-skin contact. There are a few scenes throughout the film adaptations of the trilogy in which Frodo puts the ring on his finger, either out of necessity, or clumsiness, including in the Prancing Pony, when he is stabbed at Weathertop, when he must escape Boromir on Amon Hen, and when he finally succumbs to its temptation right before it is destroyed in Mordor.
When he does put the ring on each time, he has a dangerous and painful encounter with the eye of Sauron, and each time it makes the ring harder to bear. So the chain acts as a barrier between Frodo and the evil of the ring, meaning that he doesn’t have to wear it on his finger and can transport it in a way that is less detrimental to him. It also means that other characters of the nine members of the fellowship are able to pick up the ring without being driven mad by its power, for example when Boromir picks it up out of the snow, or when Sam keeps it safe after Frodo is stabbed by the spider demon Shelob.
The second reason for the chain is one that is a little more tangible and is referenced in both the books and the films. The ring is always trying to return to its true master and has unfortunately developed some interesting qualities of its own. Not only does the ring have its own sounds and voice, feeling like a complete character within the films, it also has the cunning trick of being able to change size and abandon its wearer at will. It often enlarges itself in order to slip off the finger of its bearer and slip away to a new servant who it thinks will better serve its purpose.
This can be seen in the case of Isildur, the King of Gondor upon whom one of the Argonath is based, in which the ring betrays him and ens up in his death. Another example of this is when it escapes from Gollum in his cave during the chapter of The Hobbit ‘Riddles in the Dark’ and finds its way into the possession of Bilbo Baggins. The chain combats the ring's duplicitous nature and gives its bearer some security that it is not going to run when it senses an opportunity to deceive them.
The third special ability that the chain harnesses is a deterrent quality against the creatures that have been infected with Sauron’s will. As the chain is made from Elvish steel, it is not only incredibly strong and indestructible, it is also very hard for evil creatures to touch. A good example of this elven craftsmanship is when Frodo and Sam lead Gollum through the paths to the black gate on a chain, and he screams ‘It burns us!’ Similar can be said of when he eats Lembas bread, another invention of the elves. Therefore it follows that any creature who was to try to touch the chain in stealing the ring from Frodo would be unable to be in contact with the metal for very long before it caused them both pain and damage.
Unfortunately, it begins to have exactly this effect on Frodo himself as they draw ever nearer to Mordor. In the scene in which Frodo is captive in the tower, and the orcs have removed his Mithril chainmail, we see marks around Frodo’s exposed neck, which look like burn marks from the chain. This is in part due to the chain becoming increasingly heavy both physically and metaphorically, but also because Frodo is succumbing further and further to the ring's influence, thus the elven metal is starting to have more of an adverse effect on his skin.
Without the help of the chain, it is arguable that each of the characters in the Lord of the Rings who have ever had hold of or been offered the ring, no matter how briefly, may not have had the will or the strength to resist. The chain and its elven magic are an essential part of the war against the dominion over all of Middle-earth and is one of many elven gifts that are given to the fellowship to ensure that their quest succeeds.