Many gases, but particularly hydrogen, helium and hot air, are less dense ("lighter") than air, and so can be used for filling balloons to lift things. Hot air balloons are discussed on another page; to link to it please click here 
At sea level and a temperature of 15oC hydrogen has a density of about 0.09 kg/m3, compared with 1.22 kg/m3 for the density of air, a difference of 1.13 kg/m3.
If we contain the hydrogen in some sort of bag (an envelope), to stop it from mixing with the air, there will be an upward force (buoyancy) on the envelope. A hydrogen balloon consists of two parts: the envelope containing the hydrogen and the basket or gondola suspended from it. If the envelope and basket of a balloon, the passengers and all the other equipment weigh a total of 1000 kg, we shall need a total of about 885 m3 of hydrogen to lift it off the ground - this will fill a spherical balloon with a diameter of about 11.9 m.
From 1783, when Professor Charles made the first hydrogen balloon, until after the Second World War almost all balloons (and airships) were filled with hydrogen. Hydrogen is the commonest element in the universe and the lightest gas. It does not exist in the Earth's atmosphere because it is light enough to be able to escape from the Earth's gravitational field, but it is present in water and all organic compounds so is present in every part of all living things. It can be produced in many different ways but it is almost always manufactured by extracting it from water. It is very flammable and burns with an almost invisible pale blue flame.
Since the end of the Second World War for safety reasons helium, which is non-flammable, has almost completely replaced hydrogen for filling balloons. The safety of hydrogen balloons and airships is more fully discussed on another Page - to link to it please click here
The density of helium at sea level and 15oC is about 0.18 kg/m3, compared to 0.09 kg/m3 for hydrogen and 1.22 kg/m3 for air. The difference between the density of helium and that of air is therefore about 1.04 kg/m3, so a 1000 kg helium balloon will need about 960 m3 of helium to lift it - this will fill a spherical balloon with a diameter of about 12.2 m. A useful rule of thumb for helium balloons and airships is that at sea level 1000 m3 of helium will lift about a tonne.
Although helium is twice as dense as hydrogen its lifting capacity is only slightly less because what matters is the difference between its density and that of air. Nevertheless a balloon, and in particular an airship, designed to be filled with hydrogen will not work if it is filled with helium, although a balloon or airship designed to be filled with helium will work if it is filled with hydrogen - the Hindenburg was designed to be filled with helium although for reasons described below on this page it was actually filled with hydrogen.Helium is the second most common element in the universe, after hydrogen, as it was formed in the immediate aftermath of the Big Bang. It is also produced by nuclear fusion inside stars, including our Sun. It is the second lightest gas, after hydrogen, and like hydrogen is light enough to escape from the Earth's gravitational attraction. However it is present in the Earth's atmosphere in minute amounts, about 0.0005%, because it is produced by certain forms of radioactivity. It is at present usually uneconomic to produce it by extracting it from the air.
It was first discovered in the Sun, in 1868, by a process known as spectroscopic analysis, and named helium after Helios the Greek God of the Sun. It was not discovered on Earth until 1895, but for the next eight years it was available only in such minute quantities that it was nothing but a scientific curiosity. Then in May 1903 a company drilling for oil in a small town in the central United States hit what they thought was a natural gas (methane) geyser. But when they tried to flare off the escaping gas not only did it not catch fire it actually put out the burning bale of straw they were trying to light it with! An analysis of the escaping gas showed that it contained almost no methane, but about 12% of a gas which proved to be helium. Since then vast reserves of helium have been discovered in natural gas fields in the central United States, and to this day the United States has an almost total monopoly on helium production.
The United States Government saw the military uses of balloons and airships very early in on their development and for this reason were very careful about giving export licences for helium. Many books, and almost all web sites, on balloons and airships do not seem to know or understand this. They talk about German Zeppelins continuing to use hydrogen long after it was established that helium was safer, but there was only one reason why they did so, and that was because the United States Government would not allow helium to be exported to Germany. The Hindenburg had actually been designed to be filled with helium, and its ill-fated visit to New York in May 1937 was part of a vast propaganda exercise by Hitler's Third Reich, to convince the American people that Germany did not pose a threat to them, and that Germany was building airships for purely commercial purposes, so that the withholding of an export licence for helium was unreasonable.
Since the end of the Second World War many of the restrictions on the export of helium have been lifted and as a result helium has almost completely replaced hydrogen for filling balloons and airships throughout the whole world.
Until the 1970s almost all balloons were approximately spherical. This was because it is very easy to make a spherical balloon and very difficult to make one any other shape. Also, for a given volume a sphere has a smaller surface than any other shape so the envelope of a spherical balloon will need less material and weigh less than that for any other shape.
Today however balloons may be almost any shape. However, non-spherical balloons are usually much more expensive and are often very difficult to fly, so they are used mainly for advertising and other special purposes. You can visit a web site with lots of non-spherical balloons by clicking here
© Barry Gray June 2003
