What are these metal circles that roll on each other called? (picture)?

a href="http://www.google.com.eg/imgres?imgurl=http://www-static.weddingbee.com/wp-content/uploads/2010/03/09/img-034.jpg&imgrefurl=http://www.weddingbee.com/2010/03/10/pencils-or-goldsmiths-part-two/&usg=__JSZFUeDGGiNbIiyKMlSxdhy1jL8=&h=480&w=640&sz=84&hl=en&start=15&zoom=0&tbnid=BjmB99lpkh7NuM:&tbnh=103&tbnw=137&ei=rmI7TbqjCYOR4gbk7uGpCg&prev=/images%3Fq%3Dmetal%2Bcircles%2Brolling%2Bon%2Bone%2Banother%26um%3D1%26hl%3Den%26client%3Dfirefox-a%26rls%3Dorg.mozilla:en-US:official%26biw%3D1024%26bih%3D555%26tbs%3Disch:10%2C441&um=1&itbs=1&iact=hc&vpx=319&vpy=290&dur=1128&hovh=103&hovw=137&tx=94&ty=69&oei=o2I7TcnjMMWztAbx4rT1Bg&esq=5&page=2&ndsp=18&ved=1t:429,r:7,s:15&biw=1024&bih=555" rel="nofollow"http://www.google.com.eg/imgres?imgurl=h…/a