188 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



THE SPHERE GAP 



While the impulse spark-over voltage of the needle gaj) varies 

 with the wave shape of the impulse and is always hi<^her than the 

 GO-cyele spark over, the sphere gap is very little affected, even for 

 quite steep waves. The sphere gap thus offers a means of meas- 

 uring transient voltages over a considerable range. Figure 12 shows 

 a typical spliere gap spark, and Figure 'i.') shows that the spark-over 



220 



ZOO 



\l80 



^160 



o 



140 

 120 

 100 

 60 

 60 

 40 

 20 



O / ^3^36739/0 

 25 CA7. Spheres- Oop/n Cnjs. 



Fig. 25. — Sphere gap spark-over curve.s for GO cycles and for 

 impulse Waves Nos. 1 and 2. Wave No. 1, siiislo lialf sine wave 

 imj)ul.so ; wHve No. 2, impulso with stnep front, but with a lonir 

 tail. (Drawn curve is for G(i cycles; points are for waves Nos. 

 1 and 2, as notid.) 



voltages for a given gap do not vary over a very wide range of 

 transients. 



LINE INSULATORS 



Investigation at the lower voltages showed that the wet and dry 

 lightning spark-over voltages of insulators were equal. Tests made 

 at the higher voltages confirm these data. Typical 60-cycle and 

 lightning arcing characteristics for shielded and nonshielded insu- 

 lator strings are shown in Figures 27 and 28. The shield consists 

 of a metal ring at the line end of the string. It serves to distribute 



