Tips on building your own coherer

The type of coherer that I build is modeled on the type that Nyle Steiner K7NS shows on his website. It is simple to make and surprisingly effective. The parts are easy to come by as shown by a list of needed materials:

1/4 in. screws 1 in. long (nuts for the screws as well)
1/4 in. clear vinyl tubing
A metal grinding file, the finer cut the better
A source of metals (copper pipe, U.S. nickels, iron bolts, etc.)
Elbow grease (getting the metal power you need is hard work!)

Start by making the metal powder that you will need. I suggest that your first coherer use copper; my experience with nickel and iron show that these metals are finicky (more on this below). There are a few points to be made before you begin filing away: Use a clean, dust free piece of paper to catch the metal filings. When filing the metal use as little pressure as possible on the file. A finer metal powder will usually make a more sensitive coherer than a course powder. Though, if the powder is so fine that it sticks together and doesn't flow easily, the coherer may not operate well. Experiment, search for a happy medium!

Now, take two screws and on each thread a nut all the way up to the head of the screw. Cut a 3/4 in. length of vinyl tubing and push one screw about half way in. Pour metal filings into the tube to a depth of about 1/8 in and then push the other screw in, but not so far that it touches the metal powder. When the coherer is laid flat and shaken, the level of the metal power should be between 1/2 and 3/4 the thickness of the tube.

O.K. You have a coherer now. How do you use it? Check out the coherer circuits page for some schematics and photos of how to test your coherer. I have also built a lightning detector using a coherer.

Experience from building my coherers

My first coherer used copper powder and it worked very well. When activated its resistance would drop to 100 Ohms or less. After the coherer was tapped or shaken its resistance would be so high as to appear to be an open circuit (my digital meter won't measure resistances greater than 40M Ohms and after being decohered the resistance was greater than that). Copper powder gave a very nice on/off behavior.

The next metal tried was iron, or more accurately steel of some sort. I filed a bolt getting as fine a powder as possible and made another coherer. This one was more sensitive. It would activate at up to 6 feet using only the meter leads as antennas. The on resistance was less too, usually around 80 Ohms. There was a drawback though. This coherer was very hard to decoher! When shaken its resistance would fluctuate wildly over a wide range, 100k - 30M Ohms wasn't unusual. I wanted a more sensitive device to replace the copper coherer in my lightning detector and this behavior wouldn't work at all.

Having read that a mixture of nickel and silver powders were used in historic coherers I had high hopes for using nickel. The only ready source of nickel I have easy access to is the U.S. 5 cent piece. The composition of this coin is only 25% nickel, the rest being copper (you would never guess from the color would you?). As I had hoped the nickel coherer was very sensitive, but suffered the same problem as iron did: it decohered very poorly. Its on resistance was comparable with that of copper, but when shaken its resistance would fluctuate with wild abandon. I made several coherers using nickel, hoping to improve their behavior, and the best off resistance I measured with one was 10k Ohms! Very poor indeed.

Making a more sensitive coherer

From my experiments it was clear that nickel was the best metal to use as far as sensitivity was concerned. I could trigger a nickel coherer from 10 feet with a piezo igniter transmitter. The awful decoher resistance was a stumbling block. How to counter this bad characteristic?

I tried several things: washing and then drying the powder (I hoped the water would carry way any dust or lint fibers), making finer filings from the Nickel, and finally mixtures of different metal powders. Historic coherers used a mix of nickel and silver powders, why not try a mix of the metals I had on hand? Since I want the behavior of the copper coherer, but with more sensitivity, I started mixing the copper and nickel powders together.

After some experimenting I got a mixture that produced the results I was looking for. Using an approximate 2:1 ratio of copper to nickel powder created a coherer that was more sensitive and didn't have an off state where the resistance was quite so wild. The off resistance wasn't as high as a pure copper coherer, but when struck it decohered much better than pure nickel. My lightning detector now beeps whenever certain appliances in the house turn on!

The physical dimensions of the coherer also affect its sensitivity. There are two main measurements to consider: the length between the screw electrodes and the area that the powder covers of the ends of the screws. As the gap between the electrodes shrinks, the sensitivity increases. The more electrode area that is covered the greater the sensitivity as well. When making adjustments be careful not to leave the powder compressed, the powder needs room to move around when the coherer is struck. Also, being compressed lowers the powders resistance.

The coherer currently used in my lightning detector is my best attempt so far. I have no desire to start filing the edges off the silver dollar I have. :) The gap between the electrodes in this coherer is about 1/16 in. wide and it uses a 2:1 copper/nickel mixture. There is one other difference in this coherer. I filed the ends of the bolts flat and held the ends above a candle flame until a yellow/brown (an iron oxide layer?) color appeared. This seems to have a small effect helping the device decoher, but I need to examine this more...