ELECTROSTATICS 



41 



29. Corona. When the electrostatic stress or potential 

 gradient at some part of the surface of a conductor in air ex- 

 ceeds about 3500 volts per millimeter or 100,000 volts per inch a 

 brush discharge takes place and the air becomes conducting and 

 luminous. The discharge does not necessarily extend across 

 from the positive to the negative electrode but only exists in 

 the region where the dielectric strength of the air has been ex- 

 ceeded. Take the case of two parallel wires suspended in air; 

 the discharge or corona first appears at any rough points on the 

 wires, and finally forms a luminous envelope about them, which 

 increases in diameter as the voltage is raised. Corona causes a 

 loss of energy which increases very rapidly with increase of volt- 

 age above the critical point where the discharge begins. 



