1884.J 



Heating Effects of Electric Currents. 



465 



Atmospheric electricity has proved a great danger to insulated, 

 wires, subterranean and submarine, and to telegraphic apparatus 

 generally. Not only do the direct discharges of atmospheric electri- 

 city enter the wires, but very powerful currents are induced in 

 neighbouring wires when these discharges take place, either between 

 cloud and cloud or between cloud and earth. Various plans have 

 been devised to protect apparatus and wires from these currents. 

 Lightning protectors based on the effect of points, on the facility of 

 discharge through vacua, on the low resistance of thin air-spaces to 

 high potential, and on the fusibility of thin wires, have been used. 



The most careful and long-continued observations have shown that 

 the survival of the fittest is found, in the use of a thin flat air-space 

 A (fig. 1), supplemented by a fine well insulated protecting wire, 



B, of high resistance, wound around a brass rod in connexion with 

 earth. The air-space is obtained by superposing two smooth plane 

 surfaces of thick brass, separated from each other by a space of '004 

 of an inch, by means of a frame of mica or paraffined paper. It 

 requires (according to the researches of Messrs. Warren De La Rue 

 and Hugo Miiller*) confirmed by my own observations made in 

 Dr. Warren De La Rue's laboratory, a potential of 800 volts to strike 

 across such an air-spaCe, and when this is done a path of no resist- 

 ance, according to Faraday, is established for the atmospheric 

 discharge to flow to earth. f But destructive currents of induction are 

 often produced in telegraph wires, which have not the requisite 

 potential to strike across this air-space, and the element of time may 

 enter to force some of the current into the cable or apparatus so as to 

 * " Phil. Trans.," Part II, vol. 174. 



t Dr. De La Rue has been kind enough to repeat these experiments with the 

 following results : — 



Air 



space. 



Potential. 



001 in 453 volts. 



002 , 659 „ 



003 „ 824 „ 



00475 „ . . 1030 „ 



005 „ 1030 „ 



W. H. P., April 4, 1884. 



