1. The Gemara discusses what type of collar or patch a servant may not wear on Shabbos.
2. Collars or patches worn by servants cannot become Tamei, even if they are made of metal.
3. There are vessels that do not become Tamei because they are made of materials that are not subject to becoming Tamei.
4. There is a Halachic difference between a bell that has a clapper and a bell that does not have a clapper.
5. There is also a Halachic difference between a bell worn by an adult and a bell worn by a child.
A BIT MORE
1. The Gemara concludes that a servant may not wear a metal collar or patch that his master orders him to wear. Since it is expensive and his master orders him to wear it, if it breaks he might carry it back to his master's house and transgress the Melachah of carrying in the public domain on Shabbos.
2. Since they are not vessels that have a function and are not worn for adornment, they do not become Tamei.
3. This includes vessels made of stone, dung, and plaster (that is not baked into earthenware).
4. A bell with a clapper is considered a vessel that can become Tamei. A bell without a clapper serves no function and cannot become Tamei.
5. A bell worn by a child is made to make noise, and therefore becomes Tamei if it has a clapper. A bell worn by an adult is worn for adornment, and therefore becomes Tamei whether or not it has a clapper.