66 Mr. W. Duddell [Feb. 21, 



as the salts of the metals of the alkaline earths, into the arc. An 

 easy way to do this is to use a glass rod as a core to one of the elec- 

 trodes, the sodium from the glass making it easy to burn an arc an 

 inch or two long. 



Not only is the vapour column of the arc so sensitive to any 

 changes that may take place in the current that it can follow and re- 

 produce all the numerous vibrations which constitute articulate 

 speech, but I have also found that the intensity of the light given 

 out by the arc is also capable of following very rapid small changes 

 in the current. Thus, if the direct current through the arc be in- 

 creased and decreased rapidly even over a small range, the light 

 emitted will increase and decrease in a similar manner.* 



The light given out by one of the talking arcs varies in intensity 

 in a manner corresponding to the vibrations which form the words 

 the arc is saying, although the eye does not appreciate the fact, so that 

 if the light were allowed to fall on any apparatus or substance which 

 was sensitive to such rapid changes in the intensity of its illumination 

 and which could reproduce such variation of its illumination as cor- 

 responding mechanical movements, then it would be possible to recon- 

 vert the variation of the light into sound waves again. One such 

 substance is the metal selenium, which, when suitably prepared, has 

 the property that its electrical resistance depends on whether the 

 light falling on it is strong or weak ; in a strong light its resistance 

 is much lower than in a weak light. 



An arrangement of selenium and electrodes to show the effect of 

 light on the resistance of selenium is generally called a selenium 

 cell. 



On this basis Herr Simon | has founded a method of light tele- 

 phony ; in his method the light given out by a talking arc at the 

 transmitting station is collected by a lens or reflector and focussed 

 either directly or by means of other lens and reflectors on a selenium 

 cell placed at the distant receiving station. This selenium cell is 

 connected in series with a battery and a telephone receiver in which 

 the transmitted speech is received. The action of the arrangement 

 is as follows ; talking to the microphone at the transmitting station 

 causes the currents through it to vary, increasing and decreasing the 

 current in the talking arc ; each increase of the arc current is accom- 

 panied by an increase in the light given out, which, by means of the 

 reflectors and lens, increases the illumination of the selenium cell at 

 the distant receiving station. The increase of illumination causes the 

 cell's resistance to fall and allows the battery to send a larger current 

 through it and the telephone receiver, so that every increase of current 

 through the talking arc produces a corresponding increase in the 

 current through the telephone receiver. Thus the variations of 

 current due to the microphone are transmitted along the beam of 



* Journal of the Institution of Electrical Engineers, 1901, vol. 30, p. 236. 

 t ElektrotechniBche Zeitechrift, 1901, pp. 197 and 513. 



