TRANSACTIONS OF SECTION A. 



259 



13. On Lightning Protectors for Telegraphic Apparatus. 

 By William Henry Preece, Electrician, General Post-Office. 



For many years it was not the practice in England to protect telegraphic 

 apparatus from the injurious effects of atmospheric electricity, because the damage 

 done was so insignificant, and because the remedy was found to be worse than the 

 disease. 



But as telegraph systems increased, as the country became enveloped in one vast 

 network of wires, it was found that the damage done became considerable, until, in 

 fact, about 10 per cent, of the apparatus in use was in one year damaged. 



Lightning protectors then became essential. Many forms were tried, based on 

 the fact that when a discharge takes place through a non-conductor, such as dry 

 air, at the moment of discharge the resistance along the line of discharge is 

 practically nothing, and therefore all the charge is conducted away. According to 



Faraday, ' the ultimate effect is exactly as if a metallic wire had been put into the 

 place of the discharging particles' {Researches, Series xii., 1406). Most of those 

 tried failed. 



The survival of the fittest has been exemplified in the ' plate' protector. In this 



? lm— one °f ^ e eai 'liest introduced — one thick plate of brass is in connection with 



the earth, and another similar plate in connection with the line is placed above it, 



but separated from it by paper, or by insulating washers. The lightning, entering 



s2 



