Avian’s Blog

Yesterday at Zemanta headquarters, there was a discussion about the validity of some statistics. It went something like this:

(brought to you by the Creative Commons NC-SA 2.5 license of the original xkcd comic)

I just saw this interesting tooltip when I moved the mouse over the System menu.

I'll be spending the next week in Alexandria, at this year's Wikimania. I was at the first Wikimania in Frankfurt three years ago and it has been a pleasant experience (if I forget the desperate search for working wireless, but I guess that is mandatory at such events).

I wonder how much the conference has grown in these years. I remember Frankfurt as a nice, relatively small scale event. Halls were full of discussions about Wikipedia trivia and inside jokes (a lot of which I didn't get at that time) and Jimmy Wales was a kind of a celebrity everyone wanted to meet in person. I also recall the bad opinion I got about Richard Stallman from the way he answered questions at that conference. My impressions are still available at Cyberpipe's webpage by the way (Slovene text over English comment spam: day 1, day 2, day 3)

This year I'll be giving a 20 minute talk about Wikitag, the automatic link generation system I developed for Zemanta and a related lightning talk about an open-source MediaWiki dump preprocessor Wikiprep. So, if you're interested in the magic behind Zemanta's suggestion service and you'll be on that continent next week, you're kindly invited.

Back in 2004 I did some research into analysis of signals recorded by an instrumented tracer (English abstract). The problem I was working on involved this instrument in form of a metal sphere that you put into a river and let the water carry it just like a normal stone cobble. The tracer then measures and records the acceleration profiles of impacts with bedrock and other stones. The purpose is that from these measurements you can get some idea of what material there is in the river bed and what kind of erosion is going on there.

What I basically did is that I developed a pretty simple method of automatically recognizing the type of material that is hitting a tracer by looking at the output of three accelerometers.

To my surprise, dr. Matjaž Mikoš, one of my mentors, contacted me a few days ago with news that our work has been presented as a poster at the European Geosciences Union and the he held in invited talk about the topic at SDHR (Slovenian Association of Hydraulic Research).

(Click on the image for the full poster)

I'm really glad that the work I did turned out to be that useful.

Update: I've fixed the image so that now you can actually read the text on the poster. Thanks to Andraž for notifying me about that.

Today Slovenian highways switched from a modern contactless toll collection system called ABC to a piece of paper that you stick on your windshield and has to be manually checked by an army of inspectors.

ABC in all its tuned microstrip glory

ABC was a prepaid system where the amount you paid was proportional to the usage of the highways (well, to some approximation). The new sticky paper costs the same, no matter how much you actually drive. What is even funnier is that the speed limit through the tollgate is now 30 km/h while with ABC it was 40 km/h (and with the old system something actually had to be done while you drove through).

Welcome to back to 1950s.

This step back does have some bright sides though. One is increased privacy. Since the old system registered your tag on every entry and exit of the highway, it was possible for it to be used to track vehicles. An interesting step in the time when governments want to track citizensterrorists on every occasion. On the other hand you can already hear talks about Slovenia being the first country in Europe to adopt a new electronic system using satellite tracking, so this may not last long.

For people commuting daily by car like me the stickies also bring yearly toll expenses down to one tenth of what they were with ABC. Great for us, not so much for tourists just passing through our country and certainly not for Slovenian railways. And then people wonder why our public transport is in such a sorry state.

Back in April I posted a question about the definition of the coupling coefficient. Well, I got the answer almost immediately via email from A. B., who confirmed my suspicions of the derivation given in the book and explained to me the correct way of defining k. Now I finally got around to writing a follow up post about this (and I can also at last clear all those books from my desk).

So, the correct way of arriving at the coupling coefficient and the formula for mutual inductance would go like this (using previous flux and current designations):

Mutual inductance between coils 1 and 2 is by definition:

Due to principle of reciprocity, we can reverse the roles of coils:

Now you can define the coupling coefficient k as:

From this definition you can see that the value of k only depends on the values of mutual inductance and self inductances of both coils, which themselves only depend on the geometry (i. e. shapes and positions of coils).

Finally, from this equation it also follows that:

So what is wrong with the derivation in the book? Basically it's this single assumption, that:

and

One clear case where this equation doesn't hold is when coils 1 and 2 have the same cross-sections and are placed one on top of another (so that both magnetic fluxes through their cross-sections are equal). By varying the number of turns of both coils, you can set an arbitrary ratio between Φ₂₁ and Φ₁₁.

This example shows that the ratio between both fluxes depends on self inductance of both coils. And indeed the correct equation that follows from the definition of k is:

So, thanks again to the anonymous reader for clearing this up.

NewScientist writes about a paper that explores the possibility of using tequila as a precursor material for growing thin diamond films by chemical vapor deposition.

The Mexican authors have written this wonderful introduction, with half of the text explaining state of the art in CVD and other half explaining state of the art in tequila production.

They claim that tequila contains just the right proportion of carbon, oxygen and hydrogen for carbon atoms to bond into a nano-scale diamond coating on a silicon and steel substrate. Such diamond films are a target of research into alternative semiconducting substrates for integrated circuits.

It's a funny discovery and I would really like to know the circumstances that led to the researchers pouring alcoholic beverages into their precious equipment. From my limited experience owners of such machinery are pretty paranoid about contaminating them with unknown substances and I can't imagine someone using a chemical that isn't semiconductor grade. On the other hand they used only some 1 dl of the liquor for the experiment, so the rest of the bottle's content may have helped them with that.

I'm a bit skeptical about the repeatability of their experiment thought. I doubt that all Orendain brand Tequilas blanco have the same C-H-O atomic relationships to two significant figures (or they must use some pretty impressive distilleries).

Last Wednesday a pump in the primary coolant loop of the Krško nuclear power plant developed a leak. The operators noticed that and stopped the reactor. The leak was fixed in a few days and on Monday the plant resumed normal operations.

This relatively minor event got a disproportionate amount of media attention in Slovenia and neighboring countries, not to mention renewed fears of anything nuclear in general public.

On the other hand, I actually feel safer now.

News like this mean that the processes that are in place to shut down the plant on some abnormal event work properly. This last event is particularly interesting because of two reasons: 1) this was the first malfunction in two years and 2) human operators pushed the stop button, not some automatic fail-safe. To me, this means that we have capable people running the plant that are observant and do not fear to stop the plant when they see something unusual going on, even when the instruments say that everything is safe.

What I do fear now is that this malfunction pushed any serious chances of Slovenia getting the second nuclear power plant even further into the future.

Andraž and I are flying to 5th European Semantic Web Conference early tomorrow morning to see what is new in this field and hoping to meet some interesting people.

satellite photo of Tenerife shows some interesting geographical features (unfortunately Wikipedia says that volcano isn't active since a couple of million years ago). I hope we'll have some opportunity to see a part of this landscape first hand.

Today I wanted to burn a CD which I could listen in my car. Since the CD player there supports MP3 format this shouldn't be that hard, right? I'll just copy the music I already have on my hard disk (and which I ripped from my CD collection) to a data CD and that's it.

Well, not really. On my Debian machine, all the music got ripped into Ogg Vorbis (since that was the default setting in Sound Juicer). On my PowerBook, iTunes ripped everything into AAC (again, the default setting). I'm sure that if I had a machine running Windows, everything would be in some Microsoft proprietary format.

I know, I know. I should have checked the settings before feeding my computers all my CDs. Still, it's frustrating that I have to go through all that again.

And of course, I didn't have any such problems with music that I downloaded from the net (like free tracks from Machinae Supremacy, don't get me wrong).

A laptop on the back seat running a wireless LAN sniffer can yield some interesting results. Here's some statistics I gathered with Kismet while driving on the route Logatec - Ljubljana - Maribor - Graz:

Driving route

Percentage of networks using encryption

Channels used

Kismet has seen a total of 368 different networks.

One thing that I didn't expect to see is the channel distribution. Why are most networks on channel 11? Maybe it's a default setting for some popular equipment.

Also surprising were a couple of networks that are encrypted, but have names that include the owner's GSM number (like "041xxxxxx Call for service"). I haven't seen this before, but it seems like a nice gesture.

Unfortunately I didn't have a GPS attached to the computer, so I can't draw an exact distribution of networks on the map.

If you haven't noticed, this blog is now a bit more web-two-ohish. I've added a Foxmarks widget to the sidebar (the "Shared bookmarks" thing) that shows the last few bookmarks I made (well, those that I let you see).

Foxmarks is a Firefox plug-in and a service that allows you to synchronize Firefox bookmarks between different computers. I've been using it for some time on my desktop and laptop computers and have been very satisfied with it - it quietly lives in the background and does its thing without annoying you with any questions. I prefer it to services like delicious because it's integrated into the browser and because it allows you to use your own sync server instead of theirs if you're concerned about privacy.

They introduced the bookmark sharing service a couple of weeks ago and I thought I'll give it a try.

For a week or so I've been having terrible problems with backscatter (of the email kind, not Rayleigh). I'm receiving hundreds of mails per day to my personal email address from various misconfigured mail servers around the world, telling me that mails I have never sent didn't found their recipients. This has made my mailbox more or less useless until I implemented some filtering over the weekend (if you've sent me a mail - I'm just now going over my cleaned inbox).

It turned out filtering this kind of traffic is quite tough if you don't want to miss out genuine error reports. Not to mention that it really messed up my Bogofilter state. Since I didn't mark those error reports (which contained attached spam) as spam it learned from them and got the idea that I changed my mind and like getting Viagra offers after all.

I don't know what caused this flood of messages. It's suspicious that it started approximately at the time my mail server went down for a couple of hours 10 days ago. However I don't see how that could be related - I have a secondary mailserver in the DNS MX record, so no mail sent to addresses at tablix.org bounced or was lost.

It's also interesting what kind of garbage I got. Most common are of course the ordinary "Undelivered Mail Returned to Sender" and similar messages (usually because the mailbox is over quota). But after digging a through it I also found a surprising number of "Message you sent blocked by our bulk email filter". It looks like some companies actually reply to each and every spam message they get. Unbelievable.

Then there are more personal messages like "please forward all emails sent to this current email address to...", "thank you for contacting us about...", odd errors like "error writing message: File too large" and this gem:

Use of uninitialized value in numeric ge (>=) at /etc/smrsh/vacation.pl
line 187.
Use of uninitialized value in fcntl at /etc/smrsh/vacation.pl line 238.
Use of uninitialized value in fcntl at /etc/smrsh/vacation.pl line 252.
untie attempted while 1 inner references still exist at
/etc/smrsh/vacation.pl line 202.

So anyway, if by any chance you're running your own mail server, you should make sure that you don't do stuff like that. Read at least the FAQ about backscatter and don't use silly scripts. The first sanity check is quite easy. Fire up a telnet client and try to send mail to a bogus address. You should be rejected right after the RCPT TO step, not later notified by mail:

$ telnet mail.tablix.org 25
Connected to mail.tablix.org.
Escape character is '^]'.
220 chandra.tablix.org ESMTP
MAIL FROM: someone@example.com
250 OK
RCPT TO: foo@tablix.org
550 Unrouteable address

Sadly, this is not an April fools' day joke. I've seen this sign today outside a store in Ljubljana:

Roughly translated into English, it advertises a tachyonic protection against EM field from cell phones. In common language that means they are selling ignorant customers a very expensive piece of ordinary glass that has protective properties approximately equivalent to an empty beer bottle.

In case you haven't watched enough Star Trek (where they are a very popular plot device) Tachyons are theoretical particles that move faster than the speed of light, have imaginary mass and for which pretty much every solid theory in physics today says they can't exist. So yeah, I'll skip the glass beads and wait until they'll sell me a portable warp coil.

Last time I've read on Eee User forums some measurements that suggested that lowering CPU frequency (via the cpufreq kernel module) to conserve battery power isn't useful because the current Eee draws from the battery is the same regardless of the frequency (sorry, can't find that post now). I find that odd, but I would happily get rid of the ondemand CPU frequency governor that's making things sluggish if it really doesn't help a bit.

After some searching around the internet I found some alternative theories why lowering your CPU clock when on battery isn't good. They say that modern CPUs have the ratio of power consumption between idle and full power so high that it is better to always have the CPU at the highest clock so that it spends as little time out of idle as possible.

So today I wanted to solve this out once and for all with some measurements of my own. Unfortunately it wasn't as simple as I thought it will be. It turns out that my particular laptop will not run with an ampermeter in series with the power supply. Without the battery it starts, but is very unstable and most times resets before Debian stops booting. With the battery it doesn't even do that. Weird. I'm guessing the added inducance and resistance is causing some undervoltage protection to trigger.

Well, I'm not going to give up that easily, but this will have to wait until I can set up a more serious experiment.