DISCOVERY 



123 



The principle upon which this instrument is based 

 dep<.>nds upon the vagaries of the substance selenium, 

 an element which is closely allied chemically to sulphur, 

 with which it is often associated in nature, and which 

 is regarded as an unmitigated nuisance by the manu- 

 facturer of sulphuric acid, because it spoils his acid 

 for use in the paper-making industry. Selenium was 

 formerly obtained as a waste product from the lead- 

 chamber process of making this acid, but it is now 

 obtained almost exclusively from the electrolytic 

 smelting of copper, which is carried on chiefly in the 

 United States. The fact that is made use of is that 

 the electrial conducti\-ity of selenium in one of its 

 physical forms varies greatly in accordance with the 

 amount of light which falls upon it. This was first 

 noticed by Willoughby Smith and his assistant May, 

 who were carrying out some cable-testing experiments 

 on the Island Station Valentia, off the coast of Ireland, 

 in the year 1873. May could not get the results 

 from his instruments by day to agree with those by 

 night. He therefore came to the conclusion that 

 there must be something peculiar in the way this 

 element selenium behaved when an electric current 

 was passed through it. It had long been noticed 

 that selenium offered a very great resistance to the 

 passage of the current, but it had not been observed 

 that the degree of resistance was affected by the amount 

 Of light falling on it. It is this peculiarity of selenium 

 that is the basis of the optophone. 



Writing to a friend early in 1873, Willoughby 

 Smith tells of his discovery in rather a witty and 

 graphic manner : 



IQ09 first produced one of the up-to-date cells, and 

 Fournier d'Albe followed with his two years later. 



Presser's cell consisted of a circular piece of steatite 

 (a silicate of magnesium) covered with platinum and 

 engrav^ed in concentric rings. The whole surface was 

 then covered with molten selenium. Fournier d'Albe's 

 cell is simpler still, and far less costly. He uses a 

 small plate of unglazed porcelain about one inch 

 square; this is covered with graphite, and then engraved 

 in fine grooves from side to side, clcse together, and 

 slightly inclined, like the one in Fig. 5 below. The 

 whole is finally coated with molten selenium, with the 

 exception of a small margin on each side to serve as 

 the electrodes. These natty little structures have 

 received the distinctive name of selenium tablets from 

 their originator. They are very strong, and cannot 



" With the assistance of a microphone one can 

 hear the footsteps of a fly as loudly as if it were the 

 tramping of a horse on a wooden bridge ; but it 

 strikes me as much more wonderful that by means 

 of a telephone I can hear a ray of light falling on a 

 metal plate." 



When light falls upon selenium prepared in a special 

 way, a change takes place in its conductivity, and this 

 change occurs almost instantaneously. It is the rays 

 at the red end of the spectrum which are most efficient 

 in bringing about this change. The conducti\dty is 

 nearly double in sunUght what it is in the dark. 

 When the source of light is removed, the drop in con- 

 ductivity occurs very rapidly. 



To get the maximum effect from a small amount of 

 material, the selenium must be prepared in a special 

 form. Great ingenuity has been displayed in attack- 

 ing this problem. Graham Bell, the inventor of the 

 telephone, was the first to make a reliable selenium 

 cell, as it is called, though Werner Siemens three years 

 earlier, in 1875, had made an attempt. Presser in 



THE OPTOPHONE WITH BOOK-REST REMOVED. 



easily be damaged ; furthermore, owing to their small 

 size, they can be suitably mounted in any way for the 

 purpose in hand. It is a modification of these Httle 

 tablets that is the actuating principle of the optophone. 

 Now, if an ordinary telephone receiver be connected 

 in series with an electric battery and one of these 

 selenium tablets, a current will pass through the 

 tablet, and the current will vary as the light falling 

 on the tablet varies, since this alters its conductivity. 

 The tablet is called a " selenium bridge." When 

 flashes of light are thrown on to the selenium bridge 

 at a rate of 256 per second, the current will rise and 

 fall at that rate, and the telephone will sing out the 

 note middle C of the piano. If the pulsations of the 

 light are at half that frequency, that is, 128 per second, 

 the telephone will sing out C an octave lower, and, 

 with double the f^equenc3^ 512 pulsations per second, 

 the C one octave higher, and so on. The rate at 

 which these pulsations are sent out can be varied as 

 required, and so a note agreeable to the listener may 



