LIGHTNING PEEK 173 



this curve with atmospheric conditions. Figure 3 shows a single- 

 phase spark-over at 1,500,000 volts. 



SPHERES 



The spark- over voltage curve for spheres is given in Figure 4, 

 The values follow the equation established at the lower voltage. 



INSULATOR SPAKK-OVER 



Figure 5 gives the spark-over values for standard 10-inch disk- 

 suspension insulators. The curves are continuous from, the lowest 

 voltage to the highest voltage. 



THREE-PHASE ARCS 



Power is usually transmitted three-phase. Investigations were 

 made of three-phase arcs. The standard arrangement was to place 

 the three electrodes in the corners of an equilateral triangle. 



The pecularities of the three-phase arc and corona are that the 

 voltages are usually lower than for single-phase; the spark forms 

 in a Y if there is no dissymetry; the spark does not take place at 

 the instant of maximum voltage between conductors but at the 

 maximum voltage to neutral. A three-phase arc is shown in Fig- 

 ure 6. 



The 60-cycle alternating-current arc passes through zero and 

 maximum current 120 times a second. When the arc forms it rises 

 due to the heat. This motion, together with the alternating current, 

 causes the beautiful lace-like appearance of the arcs. The bright 

 lines occur at maximum current. 



PRACTICAL VALUE OF HIGH VOLTAGES 



In addition to its purely theoretical and scientific value just dis- 

 cussed the production of a million volts also has a practical value. 

 The immediate practical value is its use in testing 220-kv. apparatus 

 with the required factors of safety. With the future in mind it is 

 important to learn how to build transformers best suited for oper- 

 ating at voltages higher than those used at present. Even the so- 

 called theoretical investigations of the present become the practical 

 applications of the future. 



PREDATORY VOLTAGES 



The discussion so far has been limited to the useful type of volt- 

 age used in transmitting energy. The practical value of the re- 

 search was to learn how to use these voltages without loss of energy 



