308 



ELECTRICAL EXHIBITION AT PHILADELPHIA. 



tion. The rough experimental apparatus of 

 Bell looks absurdly clumsy when placed beside 

 that which experience has shown to be the 

 most convenient form. The different forms of 

 apparatus exhibited by Bell show a gradual 

 growth from the primitive conception of the 



SWAN LAMP IN SOCKET. 



principle to the present commercial form. In 

 the same space with the Bell exhibit a switch- 

 board was shown, which is capable of connect- 

 ing any one of the 3,000 subscribers .on that 

 board to any other one. Magneto call-bells, 

 exhibited by this same company, are simply 



small dynamos, the armature of which is turned 

 by the hand. This replaces the battery for- 

 merly used, which requires frequent attention, 

 and easily gets out of order. 



The microphone is also exhibited in this sec- 

 tion. But the most interesting instrument to 

 be seen here is Prof. Bell's radiophone. This 

 has been recently improved by the discovery 

 that selenium is not the only substance in which 

 electric resistance is changed when acted on by 

 an intermittent ray of light, but that other 

 substances, and some to a much greater degree, 

 are thus affected. The material that he found 

 gave the most marked variations was carbon. 

 An experimental demonstration of this quality 

 was given by means of an apparatus that trans- 

 lated the vibrations of an intermittent ray of 

 light into the corresponding sound-vibrations. 

 A plate of glass is covered with silver- foil, arid 

 this divided into two parts by removing the 

 foil along a sinuous line running across the 

 plate, the whole being coated with a film of 

 lampblack. Each half of the foil is then con- 

 nected with one pole of a battery. In the cir- 

 cuit, at some convenient place, Mr. Bell insert- 

 ed one of his receiving telephones. This done, 

 the lampblacked surface was exposed to an 

 intermittent ray of light. At the telephone a 

 note could be distinctly heard corresponding in 

 pitch to the number of vibrations of the light- 

 ray. A practical application of the principle 

 involved was effected by adding another step to 

 these already demonstrated. Instead of allow- 

 ing an ordinary ray of intermittent light to fall 

 upon the lampblacked surface, a mirror is pivot- 

 ed with a rigid arm, which is placed in contact 

 with a diaphragm of a transmitting telephone 

 in such a way that every movement of the dia- 



EDISON 8 STEAM DYNAMO. 



