Section 1: Light Conversations - Bonfires to Photophones

Light has a long history of being used to convey long distance messages.

Two examples:

• Bonfires
  Three millennia ago, the Greek victory in the Trojan War was telegraphed by lighting hilltop bonfires one after the other.
  
  But there were limitations. While a bonfire conveyed a message far faster than any messenger could run, it could only carry one piece of information - an agreed 'Yes' signal that the war was over.
• Heliographs
  A mirror flashed the sun's rays in one direction, sending a coded message. In the early 1800s, the usual code was semaphore (a system which could also operate using flags).
  
  Bonfires and heliographs shared other limitations. They were slow, unusable in bad weather and labour-intensive. Multiple observers, all within sight of each other, had to take down the message and repeat it along the chain.

With a few exceptions (heliographs are still occasionally used), light-communications began to be phased out in the 1840s, with the invention of the electric telegraph.

Reasons: the telegraph carried signals through metal wires regardless of the weather, and could have its repeater stations hundreds of kilometres apart.

Then, in 1876, came Alexander Graham Bell's telephone - and, around 1900, the radio (a method which did not even need wires, and so was called the 'wireless').

But the idea of using light for communication was never entirely shelved.

Taking the Lid Off Selenium

Just three years before Bell completed the telephone, a British firm, the Telegraph Construction and Maintenance Company, was laying a cable under the sea from Java to Darwin.

To simulate a long-distance cable, a senior electrician, Willoughby Smith, needed test material with high electrical resistance. He decided to try using selenium.

Bars of selenium were kept in a box with a sliding lid. Smith and his team discovered that the metal's resistance changed according to the position of the lid.

Selenium - The Half Metal

Jacob Berzelius, a Swedish chemist, discovered selenium (Greek: selene = moon) in 1818. Its properties place it halfway between a metal and a non-metal.

Selenium has several forms: a red non-crystalline one (the most common), a black glassy one and a grey metallic crystalline one.

It is the grey metallic form that is photosensitive.

CLOSED LID POSITION = HIGH RESISTANCE
OPEN LID POSITION = LOW RESISTANCE

Bell Enters the Picture

In Britain in 1878, Alexander Graham Bell suggested that if his telephone were placed in circuit with the selenium, a shadow might be heard to fall.

His reasoning: the telephone produced sound in response to a changing electrical current. A shadow also changed selenium's resistance - so that too, should be audible.

Willoughby Smith tried it out - and a few days later announced that he had, indeed, heard a shadow fall.

Back in Washington, Bell and his assistant Summer Tainter tried using light to produce and reproduce sound. Fifty devices later, they produced a fairly simple working model.

The Photophone

The basis of the photophone was a mirror (made of a wafer of silvered mica), mounted on a tightly-stretched canvas diaphragm. A light beam, focused on the mirror, was bounced back to a waiting listening post.

Then, when someone spoke directly into the diaphragm, two things happened.

• The diaphragm vibrated, copying the sound of the speaker's voice
• The mirror flexed and deformed, changing the intensity of the reflected light beam.

At the listening post (which had to be in line-of-sight) was a selenium cell, in circuit with a battery and a pair of Bell telephone receivers.

As the light beam's intensity changed, so did the current passing through the selenium cell. The result: a clear, recognisable sound exactly like that vibrating the mirror.

Yes, but...

While tests over 200 metres were successful, Bell's invention had some crucial shortcomings:

• The transmitter (mirror) and receiver (listening post) had to be placed within line-of-sight
• Mechanically, it was unreliable
• Fog, smoke or rain put it out of commission.

It was almost another century before voice communication by light beam became practicable.