LIGHTNING PEEK 175 



into the air and to prevent breakdown by spark-over. In addition 

 it is necessary to learn how to prevent destruction by certain preda- 

 tory voltages such as lightning. 



IV. LlGHTNINO 



A lightning stroke is generally thought of as a local but severe 

 high-voltage discharge from some cloud. As a matter of fact, the 

 electric energy that manifests itself in the flash is, in the moment 

 previous to the flasli, stored in the surrounding air for a consider- 

 able distance. The thunder cloud acts as one plate of a huge con- 

 denser, the earth as the other, while the intervening air is the 

 insulation. When the voltage between earth and cloud becomes 

 high enough, this insulation breaks down and the energy is dissi- 

 pated in the short-circuit or lightning flash. 



The electrical energy is changed into heat, light, sound, and 

 chemical energy. The light is seen in the flash, while the sound 

 is heard as thunder. Thunder is caused by air waves set up by the 

 explosive nature of the discharge. 



The chejnical efl'ects of the lightning stroke are often detected by 

 our senses in the odor of ozone that is frequently noticeable after 

 a storm. The chemical clianges occur in the path of the discharge. 

 The two main gases in the air are nitrogen and oxygen. Each 

 molecule of oxygen is normally made up of two atoms. The electric 

 field tears these apart. Some of these single atoms recombine in 

 groups of three. Ox3^gen with a molcule made up in this way is 

 called ozone. It is very active chemically because the extra atom 

 is easily detached. The nitrogen of the air is also made to combine 

 with the oxygen, producing nitrous oxide and, in the presence of 

 vapor, nitric acid. Along the discharge path are untold numbers 

 of electrons and ions — chunks of electricity moving at enormous 

 velocities. 



The voltage between cloud and ground previous to the discharge 

 causes voltage between different parts of the atmosphere, llight 

 under the cloud the voltage gradient or the voltage difference per 

 foot of air measured in a vertical direction may be very high. In 

 fact, a certain percentage of the lightning voltage exists between 

 earth and any point above. 



Lightning, the disorderly, predatory form of electricity, is dan- 

 gerous, not because of its enormous energy but because of its enor- 

 mous power and " flighty " habits. The distinction between energy 

 and power is that energy is measured in kilowatt-seconds or kilowatt- 

 hours, while power is measured in kilowatts. Kilowatts measure the 

 rate at which work is being done. If energy is dissipated in a very 

 short time the power is large and the result an explosion. 



