Lighthouses of Australia Inc. Bulletin

Denise Shultz
President, Lighthouses of Australia Inc
Photo: Denise Shultz

Lighthouse engineers and the keepers themselves experimented with different fuels for their light almost from the start. In America, there were attempts to use cottonseed oil, porpoise oil and most successfully, lard oil. French tried peanut oil and linseed oil. Olive oil was burned in Spain and Italy and in South Africa, the oil from sheep’s tail was used. These experiments failed for various reasons: some of the oils solidified at lower temperatures or thickened and deteriorated after being kept in storage, in other cases, the light produced was not good enough, and sometimes the fuels just became too expensive.

In Australia we had our own special kind of “lard” oil, made of fledglings of short tailed shearwater birds, otherwise known as mutton-birds. The fat chicks were harvested in great numbers in Bass Strait and Tasmania and their oil was used at Cape Bruny and Maatsuyker Island lighthouses where it was burned in Argand 6 wick lamps. It was not a great success though, as the oil was said to be very smoky.

But not only oils would burn and produce light. Pure magnesium metal, when burned, is also a very brilliant source of light. Who can forget the school science experiment, when you set alight magnesium ribbon. The light produced is so brilliant that it is almost impossible to look at. No wonder magnesium was also tried as a possible source of light by American lighthouse authorities. But the idea inevitably failed. It was very hard to obtain pure magnesium in such large quantities. It was also very hard to keep the magnesium wire burning without interruption for longer period of time. At the same time, large amounts of smoke and a quantity of white magnesium oxide powder were created and had to be constantly removed.

Lime light burner
Oxygen and hydrogen gases are guided through twin pipes to a cylinder of lime. When ignited, the flame causes the lime ball to luminesce and produce brilliant white light.
Diagram: Robert Icedale

Other exotic light sources, on the other hand, were much more successful and thought they did not necessarily finish inside a lighthouse, they found good use in other areas of illumination technology.

Lime light was discovered in 1820s and trialled during 19th century in Ireland, England, Scotland and also in France. It consisted of a flame produced by burning pure oxygen (O₂) and hydrogen (H₂) gases, which was trained at a small lump of calcium oxide (CaO), or quicklime. Calcium oxide is one of several chemical substances that, when exposed to high temperatures, become thermoluminescent and emit a very strong white-blue light. Even a very small piece of lime could produce thermoluminescence and, as a point source, when placed in the focus of either a curved mirror or lens, could be easily concentrated and aimed. Because all the necessary chemicals (oxygen, hydrogen and lime) could be relatively easily obtained or prepared on the spot, lime light looked like an ideal option for lighthouses.

But, despite successful trials, the lime light was finally rejected by all lighthouse authorities concerned because of several downfalls. Lime is not chemically decomposed by the flame but the high temperature to which it is exposed (around 2000°C) causes its rapid deterioration and eventual breakdown. The method for reliable lime ball exchange would have eventually been developed if it were not for other obstacles that made lime light difficult to use in remote places like lighthouses. Safe storage of highly combustible hydrogen gas was one problem that would not be solved even if both gases were produced on the site. The chemicals used to produce them (sulphuric acid, iron filings, manganese dioxide and nitrates) were either corrosive, explosive, poisonous or otherwise dangerous to be handled by untrained keepers. With the invention of electricity, the lime light option was forsaken, at least in lighthouse technology.

Lime light assembly
The gases are stored in a bladder which is put under constant pressure by hanging weights. The pressure moves the mixture of hydrogen and oxygen through the pipes to the burner where the lime cylinder is placed in the focus of the Fresnel lens.
Diagram: Robert Icedale

But the widely used phrase “ being in the limelight” reminds us that this kind of illumination found good use as a spotlight in theatres and in movie projectors.

Electric Arc became much more successful contender in the guest to maximise the light source output. Though it was invented very early in 19th century, it was not trialled as a lighthouse illuminant until 1857 in England. The luminosity of the electric arc by far exceeded that of any oil lamp commonly used at the time, but much increased performance came hand in hand with problems.

Electric arc forms between the two oppositely charged electrodes. When the electrodes are touching, electric current can flow freely through the circuit and no light is produced. As the electrodes are separated the circuit is interrupted and the current stops flowing. At the same time, voltage starts to build up. Given that the source of power is strong enough, the build up of energy at the tips of the electrodes finally results in the circuit being momentarily restored again in a form of a spark, which jumps from one electrode to another. The spark is actually highly energetic and luminous particle of ionised gas, or plasma. Many such sparks per second would jump between the electrodes.

Human eyes, of course, can not perceive such a rapid exchange and instead, see the luminous arch forming between the two electrodes. Since the electrodes were made of carbon, they quickly burned off at the tips gradually increasing the distance between them. Left unchecked, the gap would eventually become so wide that the arc would die. Inventing the device that would automatically shift the electrodes to keep their distance constant was one of the challenges that had to be overcome before arc lamps could be widely used. In a lighthouse, the arc lamp had to be truly powerful, and to keep such light burning required a powerful generator. The problem was solved with the invention of a dynamo, but it came at a price. In fact the power generation proved so costly, that most lighthouses reverted back to other, cheaper sources, like kerosene.