The Universe of Discourse

Sun, 14 Jan 2018

How do plutonium-powered pacemakers work?

I woke up in the middle of the night wondering: Some people have implanted medical devices, such as pacemakers, that are plutonium-powered. How the hell does that work? The plutonium gets hot, but what then? You need electricity. Surely there is not a tiny turbine generator in there!

There is not, and the answer turns out to be really interesting, and to involve a bunch of physics I didn't know.

If one end of a wire is hotter than the other, electrons tend to diffuse from the hot end to the cold end; the amount of diffusion depends on the material and the temperature. Take two wires of different metals and join them into a loop. (This is called a thermocouple.) Make one of the joints hotter than the other. Electrons will diffuse from the hot joint to the cold joint. If there were only one metal, this would not be very interesting. But the electrons diffuse better through one wire (say wire A) than through the other (B), and this means that there will be net electron flow from the hot side down through wire A and then back up through B, creating an electric current. This is called the Seebeck effect. The potential difference between the joints is proportional to the temperature difference, on the order of a few hundred microvolts per kelvin. Because of this simple proportionality, the main use of the thermocouple is to measure the temperature difference by measuring the voltage or current induced in the wire. But if you don't need a lot of power, the thermocouple can be used as a current source.

In practice they don't use a single loop, but rather a long loop of alternating metals, with many junctions:

A long row of conductors alternately of two different materials,
  each joined in series to the end of the next, snaking back and forth
  so that all the A-B junctions are on top and all the B-A junctions
  on the bottom.  Between the top and bottom is an insulating layer.
  The top set of junctions is heated.  Heat flows from top to bottom
  and creates a current in the series of

This is called a thermopile; when the heat source is radioactive material, as here, the device is called a radioisotope thermoelectric generator (RTG). The illustration shows the thermocouples strung out in a long line, but in an actual RTG you put the plutonium in a capsule and put the thermocouples in the wall of the capsule, with the outside joints attached to heat sinks. The plutonium heats up the inside joints to generate the current.

RTGs are more commonly used to power spacecraft, but there are a few dozen people still in the U.S. with plutonium-powered thermopile batteries in their pacemakers.

In pacemakers, the plutonium was sealed inside a titanium capsule, which was strong enough to survive an accident (such as a bullet impact or auto collision) or cremation. But Wikipedia says the technique was abandoned because of worries that the capsule wouldn't be absolutely certain to survive a cremation. (Cremation temperatures go up to around 1000°C; titanium melts at 1668°C.) Loose plutonium in the environment would be Very Bad.

(I wondered if there wasn't also worry about plutonium being recovered for weapons use, but the risk seems much smaller: you need several kilograms of plutonium to make a bomb, and a pacemaker has only around 135 mg, if I did the conversion from curies correctly. Even so, if I were in charge of keeping plutonium out of the wrong hands, I would still worry about this. It does not seem totally out of the realm of possibility that someone could collect 25,000 pacemakers. Opening 25,000 titanium capsules does sound rather tedious.)

Earlier a completely different nuclear-powered pacemaker was tried, based on promethium-powered betavoltaics. This is not a heat-conversion process. Instead, a semiconductor does some quantum physics magic with the electrons produced by radioactive beta decay. This was first demonstrated by Henry Moseley in 1913. Moseley is better-known for discovering that atoms have an atomic number, thus explaining the periodic table. The periodic table had previously been formulated in terms of atomic mass, which put some of the elements in the wrong order. Scientists guessed they were in the wrong order, because the periodicity didn't work, but they weren't sure why. Moseley was able to compute the electric charge of the atomic nucleus from spectrographic observations. I have often wondered what else Moseley would have done if he had not been killed in the European war at the age of 27.

It took a while to gather the information about this. Several of Wikipedia's articles on the topic are not up to their usual standards. The one about the radioisotope thermoelectric generator is excellent, though.

Thermopile illustration is by FluxTeq (Own work) CC BY-SA 4.0, via Wikimedia Commons.

[ Addendum 20180115: Commenters on Hacker News have pointed out that my concern about the use of plutonium in fission weapons is easily satisfied: the fuel in the batteries is Pu-238, which is not fissile. The plutonium to use in bombs is Pu-241, and indeed, when building a plutonium bomb you need to remove as much Pu-238 as possible, to prevent its non-fissile nuclei from interfering with the chain reaction. Interestingly, you can tell this from looking at the numbers: atomic nuclei with an odd number of neutrons are much more fissile than those with an even number. Plutonium is atomic number 94, so Pu-238 has an even number of neutrons and is not usable. The other isotope commonly used in fission is U-235, with 141 neutrons. I had planned to publish a long article today detailing the difficulties of gathering enough plutonium from pacemakers to make a bomb, but now I think I might have to rewrite it as a comedy. ]

[Other articles in category /tech] permanent link

Sun, 07 Jan 2018

The horn of mead

Well, yesterday I wrote an article about the drinking contest in the Gylfaginning and specifically about what was in the horn. I was very pleased with it. In the article, I said several times:

Obviously, the horn was full of mead.

A couple of my Gentle Readers have gently pointed out that I was wrong, wrong, wrong. I am deeply embarrassed.

The punch line of the story is that the end of the horn is attached to the ocean, and Thor cannot empty it, because he is trying to drink the ocean. The horn is therefore not filled with mead; it is filled with seawater.

How could I make such a dumb mistake? As I mentioned, the version I read first as a child stated that the horn was full of milk. And as a child I wondered: how could the horn be full of milk if it was attached to the sea? I decided that whatever enchantment connected the horn to the sea also changed the water to milk as it came into the horn. Later, when I realized that the milk was a falsehood, I retained my idea that there was an enchantment turning the seawater into something else.

But there is nothing in the text to support this. The jötunns don't tell Thor that the horn is full of mead. Adam Sjøgren pointed out that if they had, Thor would have known immediately that something was wrong. But as the story is, they bring the horn, they say that even wimps can empty it in three draughts, and they leave it at that. Wouldn't Thor notice that he is not drinking mead (or milk)? I think certainly, and perhaps he is initially surprised. But he is in a drinking contest and this is what they have brought him to drink, so he drinks it. The alternative is to put down the horn and complain, which would be completely out of character.

And the narrator doesn't say, and mustn't, that the horn was full of mead, because it wasn't; that would be in impermissible deceit of the audience. (“Hey, wait, you told us before that the horn was full of mead!”)

I wrote:

The story does not say what was in the horn. Because why would they bother to say what was in the horn? It was obviously mead.

No, it's not. It's because the narrator wants us to assume it is obviously mead, and then to spring the surprise on us as it was sprung on Thor: it was actually the ocean. The way it is told is a clever piece of misdirection. The two translators I quoted picked up on this, and I completely misunderstood it.

I have mixed feelings about Neil Gaiman, but Veit Heller pointed out that Gaiman understood this perfectly. In his Norse Mythology he tells the story this way:

He raised the brimming horn to his lips and began to drink. The mead of the giants was cold and salty…

In yesterday's article I presented a fantasy of Marion French, the author of my childhood “milk” version, hearing Snorri tell the story:

“Utgarða-Loki called his skutilsveinn, and requested him to bring the penalty-horn that his hirðmen were wont to drink from…”

“Excuse me! Excuse me, Mr. Sturluson! Just what were they wont to drink from it?”

“Eh, what's that?”

”What beverage was in the horn?”

“Why, mead, of course. What did you think it was, milk?”

But this couldn't have been how it went down. I now imagine it was more like this:

“Excuse me! Excuse me, Mr. Sturluson! Just what were they wont to drink from it?”

Shhh! I'm getting to that! Stop interrupting!

Thanks again to Adam Sjøgren and Veit Heller for pointing out my error, and especially for not wounding my pride any more than they had to.

[Other articles in category /oops] permanent link

The horn of milk

When I was a kid I had a book of “Myths and Legends of the Ages”, by Marion N. French. One of the myths was the story of Thor's ill-fated visit to Utgard. The jötunns of Utgard challenge Thor and Loki to various contests and defeat them all through a combination of talent and guile. In one of these contests, Thor is given a drinking horn and told that even the wimpiest of the jötunns is able to empty it of its contents in three drinks. (The jötunns are lying. The pointy end of the horn has been invisibly connected to the ocean.)

The book specified that the horn was full of milk, and as a sweet and innocent kiddie I did not question this. Decades later it hit me suddenly: no way was the horn filled with milk. When the mighty jötunns of Utgard are sitting around in their hall, they do not hold contests to see who can drink the most milk. Obviously, the horn was full of mead.

The next sentence I wrote in the draft version of this article was:

   In the canonical source material (poetic edda maybe?) the horn is full
   of *mead*. Of course it is.

In my drafts, I often write this sort of bald statement of fact, intending to go back later and check it, and perhaps produce a citation. As the quotation above betrays, I was absolutely certain that when I hunted down the original source it would contradict Ms. French and say mead. But I have now hunted down the canonical source material (in the Prose Edda, it turns out, not the Poetic one) and as far as I can tell it does not say mead!

Here is an extract of an 1880 translation by Rasmus Björn Anderson, provided by WikiSource:

He went into the hall, called his cup-bearer, and requested him to take the sconce-horn that his thanes were wont to drink from. The cup-bearer immediately brought forward the horn and handed it to Thor. Said Utgard-Loke: From this horn it is thought to be well drunk if it is emptied in one draught, some men empty it in two draughts, but there is no drinker so wretched that he cannot exhaust it in three.

For comparison, here is the 1916 translation of Arthur Gilchrist Brodeur, provided by

He went into the hall and called his serving-boy, and bade him bring the sconce-horn which the henchmen were wont to drink off. Straightway the serving-lad came forward with the horn and put it into Thor's hand. Then said Útgarda-Loki: 'It is held that this horn is well drained if it is drunk off in one drink, but some drink it off in two; but no one is so poor a man at drinking that it fails to drain off in three.'

In both cases the following text details Thor's unsuccessful attempts to drain the horn, and Utgard-Loki's patronizing mockery of him after. But neither one mentions at any point what was in the horn.

I thought it would be fun to take a look at the original Old Norse to see if the translators had elided this detail, and if it would look interesting. It was fun and it did look interesting. Here it is, courtesy of Heimskringla.NO:

Útgarða-Loki segir, at þat má vel vera, ok gengr inn í höllina ok kallar skutilsvein sinn, biðr, at hann taki vítishorn þat, er hirðmenn eru vanir at drekka af. Því næst kemr fram skutilsveinn með horninu ok fær Þór í hönd. Þá mælti Útgarða-Loki: "Af horni þessu þykkir þá vel drukkit, ef í einum drykk gengr af, en sumir menn drekka af í tveim drykkjum, en engi er svá lítill drykkjumaðr, at eigi gangi af í þrimr."

This was written in Old Norse around 1220, and I was astounded at how much of it is recognizable, at least when you already know what it is going to say. However, the following examples are all ill-informed speculation, and at least one of my confident claims is likely to be wrong. I hope that some of my Gentle Readers are Icelanders and can correct my more ridiculous errors.

“Höllina” is the hall. “Kallar” is to call in. The horn appears three times, as ‘horninu’, ‘horni’, and in ‘vítishorn’, which is a compound that specifies what kind of horn it is. “Þór í hönd” is “in Thor's hand”. (The ‘Þ’ is pronounced like the /th/ of “Thor”.) “Drekka”, “drukkit”, “drykk”, “drykkjum”, and “drykkjumaðr” are about drinking or draughts; “vel drukkit” is “well-drunk”. You can see the one-two-three in there as “einum-tveim-þrimr”. (Remember that the “þ” is a /th/.) One can almost see English in:

sumir menn drekka af í tveim drykkjum

which says “some men drink it in two drinks”. And “lítill drykkjumaðr” is a little-drinking-person, which I translated above as “wimp”.

It might be tempting to guess that “með horninu” is a mead-horn, but I'm pretty sure it is not; mead is “mjað” or “mjöð”. I'm not sure, but I think “með” here is just “with”, akin to modern German “mit”, so that:

næst kemr fram skutilsveinn með horninu

is something like “next, the skutilsveinn came with the horn”. (The skutilsveinn is something we don't have in English; compare trying to translate “designated hitter” into Old Norse.)

For a laugh, I tried putting this into Google Translate, and I was impressed with the results. It makes a heroic effort, and produces something that does capture some of the sense of the passage. It identifies the language as Icelandic, which while not correct, isn't entirely incorrect either. (The author, Snorri Sturluson, was in fact Icelandic.) Google somehow mistakes the horn for a corner, and it completely fails to get the obsolete term “hirðmenn” (roughly, “henchmen”), mistaking it for herdsmen. The skutilsveinn is one of the hirðmenn.

Anyway there is no mead here, and none in the rest of the story, which details Thor's unsuccessful attempts to drink the ocean. Nor is there any milk, which would be “mjólk”.

So where does that leave us? The jötunns challenge Thor to a drinking contest, and bring him a horn, and even though it was obviously mead, the story does not say what was in the horn.

Because why would they bother to say what was in the horn? It was obviously mead. When the boys crack open a cold one, you do not have to specify what it was that was cold, and nobody should suppose that it was a cold bottle of milk.

I imagine Marion N. French sitting by the fire, listening while Snorri tells the story of Thor and the enchanted drinking horn of Utgard:

“Utgarða-Loki called his skutilsveinn, and requested him to bring the penalty-horn that his hirðmen were wont to drink from…”

“Excuse me! Excuse me, Mr. Sturluson! Just what were they wont to drink from it?”

“Eh, what's that?”

”What beverage was in the horn?”

“Why, mead, of course. What did you think it was, milk?”

(Merriment ensues, liberally seasoned with patronizing mockery.)

(In preparing this article, I found it helpful to consult Zoëga's Concise Dictionary of Old Icelandic of 1910.)

[ Addendum 2018-01-17: Holy cow, I was so wrong. It was so obviously not mead. I was so, so wrong. Amazingly, unbelievably wrong. ]

[Other articles in category /lang] permanent link

Fri, 05 Jan 2018

Hebrew John Doe

Last month I wrote about the Turkish analog of “Joe Blow”. I got email from Gaal Yahas, who said

I bet you'll get plenty of replies on your last post about translating "John Doe" to different languages.

Sadly no. But M. Yahas did tell me in detail about the Hebrew version, and I did a little additional research.

The Hebrew version of “Joe Blow” / “John Doe” is unequivocally “Ploni Almoni”. This usage goes back at least to the Book of Ruth, approximately 2500 years ago. Ruth's husband has died without leaving an heir, and custom demands that a close relative of her father-in-law should marry her, to keep the property in the family. Boaz takes on this duty, but first meets with another man, who is a closer relative than he:

Then went Boaz up to the gate, and sat him down there: and, behold, the kinsman of whom Boaz spake came by; unto whom he said, Ho, such a one! turn aside, sit down here. And he turned aside, and sat down.

(Ruth 4:1, KJV)

This other relative declines to marry Ruth. He is not named, and is referred to in the Hebrew version as Ploni Almoni, translated here as “such a one”. This article in The Jewish Chronicle discusses the possible etymology of these words, glossing “ploni” as akin to “covered” or “hidden” and “almoni” as akin to “silenced” or “muted”.

Ploni Almoni also appears in the book of Samuel, probably even older than Ruth:

David answered Ahimelek the priest, “The king sent me on a mission and said to me, 'No one is to know anything about the mission I am sending you on.' As for my men, I have told them to meet me at a certain place.”

(1 Samuel 21:2, NIV)

The mission is secret, so David does not reveal the meeting place to Ahimelek. Instead, he refers to it as Ploni Almoni. There is a similar usage at 2 Kings 6:8.

Apparently the use of “Ploni” in Hebrew to mean “some guy” continues through the Talmud and up to the present day. M. Yahas also alerted me to two small but storied streets in Tel Aviv. According to this article from Haaretz:

A wealthy American businessman was buying up chunks of real estate in Tel Aviv. He purchased the two alleyways with the intention of naming them after himself and his wife, even going so far as to put up temporary shingles with the streets’ new names. But he had christened the streets without official permission from the city council.

The mayor was so incensed by Shapira’s chutzpah that he decided to temporarily name the alleyways Simta Almonit and Simta Plonit.

And so they remain, 95 years later.

(M. Yahas explains that “Simta” means “alley” and is feminine, so that Ploni and Almoni take the feminine ‘-it’ ending to agree with it.)

Wikipedia has not one but many articles on this topic and related ones:

My own tiny contribution in this area: my in-laws live in a rather distant and undeveloped neighborhood on the periphery of Seoul, and I once referred to it as 아무데도동 (/amudedo-dong/), approximately “nowhereville”. This is not standard in Korean, but I believe the meaning is clear.

[Other articles in category /lang] permanent link

Tue, 02 Jan 2018

Shitpost roundup, 2017-12

As I mentioned before, I have started another blog, called Content-type: text/shitpost.

The shitposts have been suffering quality creep and I am making an effort to lower my standards. I will keep you posted about how this develops. (I don't think the quality creep was the cause of lower volume this month; rather, I was on vacation for a week.)

Here is a list of last month's shitposts. I have added a short blurb to those that may be of more general interest.

I plan to continue to post monthly summaries here.

[Other articles in category /meta/shitpost] permanent link

Converting Google Docs to Markdown

I was on vacation last week and I didn't bring my computer, which has been a good choice in the past. But I did bring my phone, and I spent some quiet time writing various parts of around 20 blog posts on the phone. I composed these in my phone's Google Docs app, which seemed at the time like a reasonable choice.

But when I got back I found that it wasn't as easy as I had expected to get the documents back out. What I really wanted was Markdown. HTML would have been acceptable, since Blosxom accepts that also. I could download a single document in one of several formats, including HTML and ODF, but I had twenty and didn't want to do them one at a time. Google has a bulk download feature, to download a zip file of an entire folder, but upon unzipping I found that all twenty documents had been converted to Microsoft's docx format and I didn't know a good way to handle these. I could not find an option for a bulk download in any other format.

Several tools will compose in Markdown and then export to Google docs, but the only option I found for translating from Google docs to Markdown was Renato Mangini's Google Apps script. I would have had to add the script to each of the 20 files, then run it, and the output appears in email, so for this task, it was even less like what I wanted.

The right answer turned out to be: Accept Google's bulk download of docx files and then use Pandoc to convert the docx to Markdown:

for i in *.docx; do
    echo -n "$i ? ";
    read j; mv -i "$i" $j.docx;
    pandoc --extract-media . -t markdown -o "$(suf "$j" mkdn)" "$j.docx";

The read is because I had given the files Unix-unfriendly names like Polyominoes as orthogonal polygons.docx and I wanted to give them shorter names like orthogonal-polyominoes.docx.

The suf command is a little utility that performs the very common task of removing or changing the suffix of a filename. The suf "$j" mkdn command means that if $j is something like foo.docx it should turn into foo.mkdn. Here's the tiny source code:

    # Usage: suf FILENAME [suffix]
    # If filename ends with a suffix, the suffix is replaced with the given suffix
    # otheriswe, the given suffix is appended
    # For example:
    #   suf baz    => foo.baz
    #   suf foo     baz    => foo.baz
    #   suf        => foo
    #   suf foo            => foo

    @ARGV == 2 or @ARGV == 1 or usage();
    my ($file, $suf) = @ARGV;
    $file =~ s/\.[^.]*$//;
    if (defined $suf) {
      print "$file.$suf\n";
    } else {
      print "$file\n";

    sub usage {
      print STDERR "Usage: suf filename [newsuffix]\n";
      exit 1;

Often, I feel that I have written too much code, but not this time. Some people might be tempted to add bells and whistles to this: what if the suffix is not delimited by a dot character? What if I only want to change certain suffixes? What if my foot swells up? What if the moon falls out of the sky? Blah blah blah. No, for that we can break out sed.

Next time I go on vacation I will know better and I will not use Google Docs. I don't know yet what instead. StackEdit maybe.

[ Addendum 20180108: Eric Roode pointed out that the program above has a genuine bug: if given a filename like a.b/c.d it truncates the entire b/c.d instead of just the d. The current version fixes this. ]

[Other articles in category /Unix] permanent link