i64 NIKOLA TESLA 



piclity, improving vision; the ears dry out and become more 

 susceptible to sound. Objects can be clearly distinguished 

 there at distances such that I prefer to have them told by 

 someone else, and I have heard — this I can venture to vouch 

 for — the claps of thunder seven and eight hundred kilometers 

 away. I might have done better still, had it not been tedious 

 to wait for the sounds to arrive, in definite intervals, as 

 heralded precisely by an electrical indicating apparatus — 

 nearly an hour before. 



In the middle of June, while preparations for other work 

 were going on, I arranged one of my receiving transformers 

 with the view of determining in a novel manner, experiment- 

 ally, the electric potential of the globe and studying its 

 periodic and casual fluctuations. This formed part of a plan 

 carefully mapped out in advance. A highly sensitive, self 

 restorative device, controlling a recording instrument, was 

 included in the secondary circuit, while the primary was con- 

 nected to the ground and an elevated terminal of adjustable 

 capacity. The variations of potential gave rise to electric 

 surgings in the primary; these generated secondary currents, 

 which in turn affected the sensitive device and recorder in 

 proportion to their intensity. The earth was found to be 

 literally alive with electrical vibrations, and soon I was deeply 

 absorbed in this interesting investigation. No better opportu- 

 nities for such observations as I intended to make could be 

 found an3^where. Colorado is a country famous for the natural 

 displays of electric force. In that dry and rarefied atmos- 

 phere the sun's rays beat the objects with fierce intensity. 

 I raised steam, to a dangerous pressure, in barrels filled with 

 concentrated salt solution, and the tinfoil coatings of some 

 of my elevated terminals shriveled up in the fiery blaze. An 

 experimental high tension transformer, carelessly exposed to 

 the rays of the setting sun, had most of its insulating com- 

 pound melted out and was rendered useless. Aided by the 

 dryness and rarefaction of the air, the water evaporates as in 

 a boiler, and static electricity is developed in abundance. 

 Lightning discharges are, accordingly, very frequent and 

 sometimes of inconceivable violence. On one occasion ap- 

 proximately twelve thousand discharges occurred in two 



