This may be a completely dumb-ass question, but so be it. Helium balloons float because they have a lower density than air. Well, doesn’t a vacuum tube (like in old computers) have a MUCH lower density? Shouldn’t it float?
Is it because the glass and stuff is just too heavy? In a related question, does the thickness of the glass required go up as the volume of the vacuum increases? Could you make a blimp-sized thing with a vacuum instead of helium?
“Is it because the glass and stuff is just too heavy?”
Yes. The density of air is only about 1.2 grams per litre. The buoyancy of a good size tube would be less than 0.01 ounces. The glass, on the other hand, probably has a weight of 2 ounces. This leaves a net weight of 1.99 oz.
Yes, if you had a large, rigid container with a vacuum in it, it would float. So long as the overall density was less than air.
There is a substance called “Aerogel”, IIRC, that is like a sponge but made up of millions of tiny vacuum bubbles. IIRC, this stuff will just barely float, or have neutral bouyancy in air.
Who says they don’t float??
Many vacuum tubes aren’t actually a total vacuum. They are sometimes filled with an inert gas at low pressure.
Yes, if you could find a way to keep it from being crushed. Francesco de Lana thought of the same thing over 300 years ago. He wanted to evacuate the air from copper spheres and use them to float his airship.
Not vacuum bubbles, but air bubbles. Aerogels are 99.8% air.
Cite: http://stardust.jpl.nasa.gov/spacecraft/aerogel.html
Could we make a vacuum foam? It would be really handy to have something lighter than air that you could just cut to fit…
I know that foams are initially bubbled up by gas pressure, so it wouldn’t be a one-step process. Perhaps something like this:
Can we foam metal?
… have it immediately crushed by the ambient pressure.
Materials are constantly being improved, maybe someday.
True enough, though relatively rare (voltage regulators), but regular ole’ vacuum tubes float nicely. Displacement!
I think the OP meant in the air.
I would hate coming home to my amp just to find the tubes floated out of their sockets and are against the ceiling.
None of these things are true vacuums since a vacuum contains nothing and these would at least have some molecules from whatever the interior is made out of. Or at least ive always thought…
Obviously it’s not an absolute vacuum, but it’s good enough vacuum for it not to float.
JZ
Yes, but it’s not easy to do and have it be very strong. Really cheaply done metal castings are filled with gas pockets and tend to be rather weak.
However, NASA once did an experiment on casting metal in micro-gravity conditions and found that metals cast under such conditions tend to be lighter and stronger than metals cast on Earth. So, if we can ever find a cheap way off this rock, we might be able to do it easily.
The bouyancy of a vacuum is only slightly greater than that of hydrogen or helium. Molecular hydrogen has 7% the mass of air, and helium 14% the mass of air. But the vessel holding the gas is sustained by the gas pressure, which equals that of air. This means that a vessel holding gas will always be lighter than one holding a vacuum (net of the contents), since the vacuum vessel will have to be rigid and will have to be at least as good at preventing gas diffusing in as the gas vessel.
(And have you ever seen a vacuum tube? There’s a lot of metal and glass and often very little vacuum.)
Yeah, how do you know they dont float? I have lots of them, saved them in case I need to have my old antiques repaired and the repairman needed parts, but I never tried putting any of them in water. Old tubes are getting harder and harder to find, and none of the drugstores have tube testers anymore.
Wait a minute…
Is the OP talking about floating in air instead of water? If so, he may be getting lack of air confused with lack of gravity. There was another thread based on this premise a while back. Not an easy notion to dispel when the movies reinforce it. (One of those asteroid impact movies from '98 had astronauts floating around in a giant vacuum chamber to train for a space mission).
My first impression was thinking a vacuum wouldn’t have any bouyancy, particularly after the OP bringing up helium balloons had me thinking of those and hydrogen balloons and hot-air ballons, and those all rise in the atmosphere, and I’ve never heard of vacuum ballons. That threw me off, I should have been thinking about bouyancy. According to refusal a vacuum has even a slightly better bouyancy than hydrogen or helium. I just took the glass part out of my thermos bottle, and it definitely floats. Evidently, the OP thinks the vaccuum tubes will not float. Perhaps some will and others won’t depending on how heavy the glass tube is, and as others have pointed out, some are filled with a gas. How many people have seen TV’s float? Is that because the big tube is mostly a vaccuum? Yes, according to this site: http://www.zyra.org.uk/tvfloat.htm
No wonder the OP doesn’t answer. Look at the date of the OP: 12-22-00! Somebody had some free time on their hands this Labor day weekend to raise this one back from the dead, eh Tedster? 
JZ
You’ll never be able to get an evacuated balloon to float, for exactly the reason that Berkut implied: the materials just ain’t up to it.
Consider a spherical balloon, which is the strongest shape for its weight. The total weight of a hollow sphere is:
Weight = [symbol]r[/symbol][sub]mat’l[/sub]*(4[symbol]p[/symbol]R[sup]2[/sup]t)
where [symbol]r[/symbol][sub]mat’l[/sub] is the density of the material you’re making the balloon out of, R is the radius of the sphere, and t is its thickness. Likewise, the buoyancy of the evacuated sphere is
Buoyancy = [symbol]r[/symbol][sub]air[/sub]*(4/3[symbol]p[/symbol]R[sup]3[/sup])
To float, the sphere has to have more buoyancy than weight; i.e., buoyancy/weight > 1. An easy calculation:
Buoyancy/weight = ([symbol]r[/symbol][sub]air[/sub]/[symbol]r[/symbol][sub]mat’l[/sub])*(R/t)/3 > 1
So pick any material, and you can figure out how thin you have to make the spere walls (R/t, in other words) for it to float.
Now look at the other side of the coin: how thin can you make the walls of a sphere before it breaks? A sphere can break for two reasons: either the stress tears it apart, or it buckles. Consulting my reference book, I find two equations. To prevent overstressing,
(R/t) < 2[symbol]s[/symbol]/P
where [symbol]s[/symbol] is the material yield stress and P is the pressure (~14.5 lb/in in this case). To prevent buckling,
(R/t) < Sqrt(0.37E/P)
where E is the Young’s modulus of the material. In general, the second criterion is the hardest to satisfy. Substituting in values for, say, carbon fiber (E = 33,000,000 psi and [symbol]s[/symbol] = 500,000 psi) gives (R/t) < 69000 for the first criterion and (R/t) < 917 for the second.
Substituting the density of carbon fiber (108 lb/cubic ft) and air (0.075 lb/cubic ft) into the first equations above gives
Buoyancy/weight = 0.075/108/3*(R/t) = (1/4320)*(R/t)
To be buoyant, then, a carbon fiber sphere must have (R/t)> 4320. But, to avoid buckling, (R/t) < 917. Whoops. Big difference.
You can possibly do slightly better with another material (but I don’t know what). You also might be able to do better by adding in some struts to brace the sphere and keep it from buckling… but then of course you’re adding in more weight.