Sunday, November 23, 2014

My first coherer

Recently the topic of the meeting of the Utah Amateur Radio Club was on "old" radio.  Really old radio, which is to say "radio" in the days before vacuum tubes and even crystal detectors.

One of the detectors used in these old days was the Coherer (see this link.)  "Discovered" more or less by French physicist Eduoard Branly in the 1890 and based on phenomenon observed by others, this device does not detect the presence of radio signals in the "conventional" way, but it goes from a high resistance state to a low resistance state in the presence of a radio signal - and then it needs to be "reset" (by mechanically tapping it) before it can detect again - a short of "one-shot" detector.

The most common type of detector (the "Branly" type) consists of metal filings located between two conductors, typically in a glass tube.  As it turns out these filings can be different types of metals, often described as "imperfect" conductors", but one should not confuse this as meaning that these are "semiconductors" like diodes, but rather referring to (possibly) semi-oxidized bits of metal of various sorts.

Back in the heyday of the coherer - before it was made obsolete in 1907 with the advent of the crystal and electrolytic detectors (yes, the "crystal" detector was a vast improvement over the coherer, which tells you how "bad" it actually was!) the most common type of transmitter was the spark gap which emitted a short, high-energy repetitive burst of radio signal.

How, exactly, a coherer works is still not well understood even today, but it is believed that there may be some sort of "micro-welding" of the particles on the molecular level in the presence of an RF signal that causes them to form a path that will carry DC that, after detection, is then broken (literally!) by tapping the coherer, mechanically, with a device called a "de-coherer" that rearranges the particles within.

Making a coherer:

There are a number of articles to be found on making a coherer, but nothing terribly specific that I found on the rather short notice caused by my rush to build a coherer:  The club meeting was later that evening and I was in a bit of a hurry to throw together the coherer at the last minute.  I found a web page by Neil, the "  " that included a portion on making a coherer that said, in effect that about any sort of metal filing that he'd happened to use did work, at least after a fashion - although some things worked better than others.

So, I went to work.

A few weeks earlier I'd ordered some inexpensive 25mL glass burettes from American Science and Surplus and from one of these I cut up a few lengths, each about 2-3/4" (approx. 7cm) long using a diamond cutoff saw and flame-polished the sharp edges with a mapp-gas torch - but just enough to make it safe without causing the diameter to narrow.

Standing one of these the pieces of burette on a square of cardboard I placed a number of short pieces of flux-free, silver-bearing, lead-free plumbing solder and then, carefully (!) heated the glass with a propane torch.  Since the burette was made from "Pyrex" type glass, it was not likely to shatter and soon, the molten solder filled the piece of glass - but it took 3-4 times before I managed to get it right:  I either knocked it over or, in its liquid state, it would run out of a small gap in the bottom!  Eventually, I managed to carefully press it against a paper towel to seal the bottom to keep it from leaking out and I soon had a "plug" of solder the same diameter as the inside of the glass burette!

After this had cooled I wrapped it in a paper towel and then proceeded to (carefully!) break free it from the glass burette by hitting it with a wrench, liberating the metal plug within.  From that, I cut away the dross that had formed at what had been the top end and in the middle, cut it diagonally, polishing the two pieces with small needle files once I was done.
Figure 1:
My first coherer, of the Branly type, made from a piece of a glass burette, solder, filings from contacts of
defunct circuit breakers and some epoxy!  It works, but not very well!

At this point I had to reduce the diameter of these two pointed "slug" ends slightly so that they would fit easily into the other piece of burette that I'd cut:  If I'd attempted to force it into either end of the burette piece, it would have surely shattered the glass!  Once I had two pieces that I could slide into at least one end of the burette segment, I then soldered short pieces of wire to the flat sides of these two slugs using a very hot iron and pre-tinned wire with 60/40 solder:  A bit of skill was required since I was soldering solder to solder!

Having done that I then set to "make" the filings that go in between the two electrode "plugs" that I'd just prepared.  In other reading I'd found accounts that indicated that nickle-silver coins made good filings so I proceeded to attack the contacts of some defunct circuit breakers that I'd rounded up with some needle files.  Working over a piece of white paper, after about 20 minutes and three pairs of contacts I had a nice pile of filings ready.

One of the pieces of glass that I'd reserved was the "bottom" end of the burette - the one with the nipple - for the body of the coherer itself and I threaded with wire through that end.  Mixing some 5-minute epoxy I worked it into the hole and moved the plug back and forth so that it made a complete seal around the plug to prevent any filings from finding their way around its sides and then stood it on end to prevent the epoxy from running out.  Very carefully "hitting" the assembly with a bit of heat from the propane torch, the epoxy hardened almost instantly - a fact tested by poking it with a piece of wire - and I poured the filings into the open end.  Carefully lowering the other electrode/plug I oriented it so that the two "slopes" of the electrodes met toward the middle and then put more epoxy into the other end to seal it as well - and waited for the epoxy to set.

After 15-20 minutes, I carefully applied a bit of heat which accelerated the curing of the epoxy to the point of it being safe to handle and I now had a completed coherer, ready for testing.

Or so I thought...

The way that a coherer is supposed to work is that its DC resistance is supposed to be very high (if not close to infinite) until the presence of a fairly strong RF signal, at which point its DC resistance will drop - and stay that way - until it is reset by being mechanically disturbed.
Figure 2:
An old Ford coil being used as the source of the "Spark Gap" transmission.

In this case, whenever the filings were between the two conductors, I was getting just a few ohms of resistance, essentially making it useless as a detector.


With very careful fiddling I discovered that I could arrange the filings so that they barely touched one electrode or the other and then, if I set off a spark on the nearby Ford coil, the resistance would go from, say, 40 ohms to 20 ohms - enough to demonstrate that it did work, but not very well.

With the demonstration at the club meeting coming up in a few hours I figured that this would have to do, so I set it aside for a while to do other things, but when I came back in an hour or so I found that I could much more easily arrange the filings and now get a few hundred ohms, trigger the spark, and then get a few ohms or tens of ohms:  The only thing that I can figure is that the freshly-filed, silver-laden particles had somewhat tarnished after that time and the "imperfect" connection was now (slightly) easier to attain.

Figure 3:
Diagram of the coherer-based detector.  Not shown in this diagram are the particles between the sloped
electrodes of the coherer, or the "de-coherer"!

Using the coherer I wired up the circuit shown in Figure 3, above.  This has an LED in series, putting only about 1.1 volts across the coherer itself with a pair of 1k resistors "isolating" the the device at RF, allowing the wires connecting it to act as sort of an antenna if desired.  For determining if the coherer had been triggered, one could look at the LED's brightness and/or monitor the voltage drop across one of the 1k resistors with either a voltmeter or and oscilloscope.  When "setting up" the coherer it was a matter of fiddling about with it until the red LED lit dimly.  When the spark "triggered" the coherer, the LED would then illuminate much more brightly until the glass body of the coherer was lightly tapped with the plastic body of a ball-point pen, reverting back to its "dim" state.

When it came to demonstrating the operation of the coherer at the club meeting I simply connected my digital oscilloscope across one of the 1k resistors.  As it turns out, my 'scope has a VGA output that I connected to the club presentation room's overhead projector so that everyone present could see the trace on the 'scope change DC level with triggering of the coherer when it detected RF from the sparking of the nearby Ford coil, and then be reset when I "de-cohered" it by gently whacking it with a ball-point pen!

* * *

How sensitive was this coherer?

I never did try to check the range.  The "transmitter" was just the Ford coil in Figure 2, just as depicted with the two short #12 AWG wires connected to it being the "antennas", but the coherer seemed to reliably trigger at a distance of about 6 feet (2 meters) which was the maximum distance that we tried being that this was the size of the table on which we'd set up our demonstration!

Considering the size and shape of the wires in Figure 2 I figure that the this spark gap transmitter is actually operating in the VHF/UHF/Microwave range, much like that of the original Hertizan Wave apparatus of the late 19th century!  - see this link.

So there you have it, a coherer!

* * *

It's been a few weeks now since I built this coherer and its resistance is still too "low" in that I have to fiddle with it to to get it to work, but it does seem to be a bit less "finicky" in getting it set up to be triggerable thanit was when I'd first built it, so whatever is going on with those metal particles to make them work better still seems to be happening, albeit much more slowly!

At some point I may try building another one or two coherers using different materials, once I have done a bit more research on the topic (I still have some pieces of burette left!) but that will have to wait for another day!


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