170 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



1,000,000 3-phase 60-cycle root mean square values.^ It Avas of inter- 

 est to find if the laAvs aiul curves established at the lower voltatjes 

 still applied at the very high voltages. The results of some of the 

 very high 60-cycle tests will now be described. 



CORONA 



When voltage is applied between certain types of conductors, as, 

 for instance, parallel wires a considerable distance apart compared 

 to their diameters, and is gradually increased, a crown of light sud- 

 denly bursts out around the conductors at a very definite critical 

 voltage. The light is accompanied by a hissing noise. This is called 

 corona. As the voltage is increased, the corona extends farther and 

 farther out until finally a spark extends from metal to metal. The 

 air is said to be ionized in the corona brush. The sudden outburst 

 occurs when the voltage gradient at the conductor surface, where 

 it is greatest, is sufficient to bring the ions and electrons up to suffi- 

 cient velocity in their mean free path to produce others by collision 

 with atoms. Certain chemical changes take place in the air in the for- 

 mation of ozone and nitrous oxides. Corona and brush discharges are 

 the same phenomena. In fact, corona and spark-over are also the 

 same. The brush or corona are sparks to space. On relativel}'^ large 

 electrodes placed close together, the intermediate corona can not form 

 and the first evidence of over stress is a spark between conductors. 

 The reason for this is known. ^Vitli certain conductor configurations 

 any increase in the conductor diameter tends to reduce the stress. 

 The corona extends out and stops when the diameter is such that the 

 stress is below the breakdown stress. With other tj'^pes of electrodes, 

 an increase in diameter tends to increase the stress, A brush once 

 started must, therefore, extend directly between electrodes. The 

 first type is changed into the second type with increasing voltage 

 and the resulting extension of corona. 



Corona starts at a very definite voltage. Several years ago a 

 law was established for calcvdating this critical starting voltage 

 for parallel wires based upon tests up to 2.50,000 volts. The factors 

 determining the critical voltage are diameter, spacing, roughness 

 of surface, and tem[, rature and barometric pressure. The diameter 

 of the conductor is the most important factor, A hnv was also 

 established to predetermine the loss caused by corona. There is 

 no appreciable loss until the critical voltage is reached. The loss 

 then increases as the s(|uare of the excess voltage above the critical 

 voltage. 



'The single-phase tests have just recently (Oct. 8, 1925) been carried to approximately 

 2,200,000 root mean square or over 3,000,000 crest. 



