LIGHTNING CONDUCTORS. EARTH CURRENTS. 95 



disturbing. In passing from level to high land currents 

 of varying direction often traversed the communications 

 and impeded the working of the apparatus. A late 

 autumn storm caused widespread destruction, which 

 led me to construct lightning-conductors for the pro- 

 tection of the lines and apparatus. In order to ascertain 

 the most efficient form of lightning-conductors 1 set 

 up between two parallel wires spikes, balls, and plates 

 at equal intervals from one another, and observed the 

 sparks, caused by the discharge of a large battery of 

 Leyderi jars, which passed between these three adjacent 

 lightning - conductors. It appeared that very weak 

 discharges took their course solely through the spikes, 

 whilst stronger ones passed mainly between the balls, 

 and very strong ones were carried off in a large 

 number of sparks almost entirely through the plates. 

 For actual lightning therefore roughened metallic plates 

 placed nearly opposite one another proved particularly 

 effective. The influence of the Northern lights also 

 made itself frequently perceptible, and at times in a very 

 disturbing degree, especially in the underground line 

 running mainly from east to west. Thus during the 

 great aurora in the autumn of 1848 communication 

 was interrupted for days between Berlin and Cothen 

 on account of violent rapidly changing currents. This 

 was the first observation of the connection between 

 earth currents, magnetic disturbance, and the aurora 

 borealis. 



When the underground line had been extended 

 to Erfurt, Halske's watery by-passes were no longer 



