Coherer
What is a coherer?
The first practical device used to receive radio waves was a simple electro-mechanical device called the coherer. Edouard Branly is credited with discovering and creating the first coherer in 18901. In only a few years, several other physicists began using and improving the coherer to the point that Marconi used it in popularizing the new technology of "wireless".
New technology always supplants old and the coherer is no different; it was only used for about ten years before better electromagnetic wave detectors were developed. I'm writing about this old technology because it offers some interesting opportunities for the amateur scientist. The way a coherer behaves is easy to understand (why they operate as they do is another matter) and anyone with modest tools and budget can build one.
Still you may question why to build a coherer. Well, a good reason is that they are still useful! I have used the coherer as the basis of an activity for high school students (it's very hands on!) and as the main element in a lightning detector. If you have an interest in materials science or just like to tinker, many different metals can be used in the construction, and different metals can affect how sensitive the coherer is to electromagnetic radiation. There is a lot that can be investigated!
Making a coherer
A coherer is simply two metal contacts that have a metal powder between them. The design I use was developed by Nyle Steiner K7NS. At the height of their popularity a coherer was a fairly sophisticated device: the contacts were made of silver, the metal powder was a mixture of nickel and silver, and the coherer was contained in an evacuated glass tube2.
A very effective coherer can be made without much effort (no evacuated glass tubes!). Figure 1 shows the basic design used by Nyle Steiner.
Larger image
This coherer is simply two 1/4 inch threaded screws holding a metal powder in a short length of 1/4 inch vinyl tubing. What simplicity this is! It's easy to make (if you use copper powder it is almost foolproof) and a high school student will not be mystified by it.
So far I have tried using three different metal powders in my coherers. At the moment I like copper powder best; it is almost foolproof to use. The two others I have tried are iron and nickel. (Well, neither were quite pure. The iron came from a steel bolt and the nickel from a U.S. nickel.) Oh, I left out one metal powder I tried: aluminum. Aluminum didn't work at all, but the reason could be because the aluminum powder I used was the bulk chemistry supply type and it could have been too course or had a heavy oxide coating on the particles.
To continue this discussion, see the links below. I have two pages discussing how I think coherers operate and my experience with different materials.