Avian’s Blog

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.

Today I had the opportunity to play with an USB HSDPA modem. This is the latest generation of the packet data connection through a cellular network which can in theory get you a 7.2 MB/s downlink. You get it in a soup can in combination with some subscription plans at Mobitel.

The modem itself obviously has several names, depending on who you trust:

• The advertisement says it's "GlobeSurfer iCON 7.2",
• the front side of the modem says "Option",
• the back side says "Qualcomm 3G CDMA model GI0205"
• and finally lsusb says it's "Globetrotter HSDPA Modem".

It certainly gives you the warm feeling that various marketing departments are working together to reduce the confusion here. Just for the record, USB ID is 0af0:6911.

Anyway, the thing works without problems on Linux. It is used as a standard serial modem with a PPP connection, except that you need to give it some special AT commands first.

When I plugged the modem in I got three USB serial devices: /dev/ttyS0, ttyS1 and ttyS2. First two didn't respond to any AT commands, while ttyS2 responded with the usual OKs. I didn't need to do any extra steps to switch the hardware from the simulated CD mode to modem mode as some people reported.

The actual configs I used are here. I don't know why, but sometimes two attempts are necessary to get a connection.

# pppd configuration (e.g. /etc/ppp/peers/qualcomm)

# Change if using with some other mobile provider
user "mobitel"

connect "/usr/sbin/chat -v -f /etc/chatscripts/qualcomm
/dev/ttyUSB2
115200

noipdefault
usepeerdns
defaultroute

noauth
crtscts
passive
modem
idle 7200

# No support for compression
noccp
novj

# chat script (e.g. /etc/chatscripts/qualcomm)
ABORT		BUSY
ABORT		VOICE
ABORT		"NO CARRIER"
ABORT		"NO DIALTONE"
ABORT		"NO DIAL TONE"
TIMEOUT		5
""		ATZ
OK 		ATE1
OK		AT+CPIN?
# insert your PIN here
READY\r\n\r\nOK-AT+CPIN=your_pin_here-OK	AT
OK		AT+CGDCONT=1,"IP","internet","0.0.0.0",0,0
OK		AT+CGDATA="PPP",1
CONNECT   	""

You also need to enter a proper username and password into chap-secrets file (PAP authentication isn't supported). For Mobitel this pair works:

# Secrets for authentication using CHAP
# client	server	secret			IP addresses

mobitel		*	internet		*

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 week Zemanta released Re-blog - a feature that allows easy and proper quotation by clicking on a small icon on the bottom of a blog post.

I found the previous Zemanta icon visually annoying and did not want to include it in posts on my blog. However, in the spirit of "don't just complain, but suggest a solution", I started up GIMP and came up with these in an hour or so:

Compare this to the old icon:

When I got back from Tenerife I was surprised to learn that my "Traditional" version of the icon is now one of the icons you can choose in the preferences page.

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.