A silent butler is a container with a handle and a hinged cover, used for collecting table crumbs or emptying ashtrays. It may or may not have feet on the bottom. The butler would make his way around the table with this container and a small brush and remove crumbs from the table. (If you watch Downton Abbey, pay close attention at the meal scenes.) The differentiating factor between a silent butler and a bed warmer is the length of the handle. These are not widely used anymore and are a collector's item.
From Wikipedia, "was a common household item in countries with cold winters, especially in Europe. It consisted of a metal container, usually fitted with a handle and shaped somewhat like a modern frying pan, with a solid or finely perforated lid. The pan would be filled with hot coals and placed under the covers of a bed, to warm it up and/or dry it out before use. (Wikipedia.) The bottom is usually flat without feet so that the pan can easily be run between sheets when warming the bed. After the invention of rubber, bed warmers were replaced with hot water bottles. In the 20th century, the electric blanket came along.
So there you have it. Feel free to post this article in your groups when the silent butler vs bed warmer argument appears!