Suppose I need to pronounce the numerical value of Planck’s constant in SI units, 6.63·10[sup]-34[/sup] (or 6,63·10[sup]-34[/sup] in French notation, I suppose). In English I’d say “six point six three times ten to the negative thirty-fourth power.” What would I say in French? My best guess is “six virgule six trois fois dix à la puissance moins trente-quatre”. Is that the correct and conventional way to say it? Should it be “fois” or “multiplié par” or doesn’t it matter?
If this isn’t just intelelctual curiosity and you also need to write the numbers, please use an apostrophe. It’s called an “upper comma” in many European languages and this lead to computers having it as a lower comma… but it should be an apostrophe. Also, that dot for multiplication is not the way I’ve ever seen another European write it.
6’63x10[sup]-34[/sup]
I think the way you say it is correct but a bit overdone, never heard any French say the “puissance”. More like “six virgule six trois fois dix à la moins trente-quatre”. Yes, I know what’s “to the -34” isn’t the 6’63 but the ten, but people speak in shorthand.
My French sucks, so if any of my neighbors to the north has corrections, please step up, I need to learn more French 
In France they don’t use an “upper comma” as far as I know. I’ve always seen the decimal point as a comma on the line, and the “upper comma”, if used at all, is used to separate thousands, like 4’999’999,95. Although the usual way is to use a space between groups of three, like 4 999 999,95.
As for the pronunciation of the exponent, my French dictionary says it would be “dix puissance moins trente-quatre”, with no “à la”.
Until now … I had thought that French speakers would express the number 10.5 as “dix point cinq” – with the word point pronounced, of course, as /pwã/. Is that usage unknown and incorrect?
If something was displayed as “10.5” then French speakers (Quebec) would probably say “dix point cinq”. For the most part, though, if they had to write it themselves, they would write “10,5” and say “dix virgule cinq”. In Quebec, both are common, thanks to the heavy English influence of using a dot “.” to display decimals. I’ve never seen upper comma used to separate thousands. For large numbers, francophones here would use spaces, e.g. 198 898,98 for 198,898.98 more common in English. Though it is also common to see spaces in the English usage too: 198 898.98.
Now I’m curious… I’m about to start a lab job in Quebec (having only worked in Ontario before) and I don’t know which of the two will be more common. I wonder if I’ll have to change my ways?
Well, the official use in either language in Canada is to use SI notation: space to separate thousands, and either comma (French) or point (English) at the radix. At least that’s what I was taught in school in Ontario.
Can someone please explain to me why I thought he was talking to French in France? Oh wait… he used a comma. OK. I’m feeling a tad less dumb than a minute ago. Where are our French-from-France users, I know we had some…
I would go with “six virgule soixante-trois fois dix exposant moins trente-quatre”. But your version is just as correct.