1920.] Berks and Ferguson.—High-tension Insulators. 187 
nesses as the high frequency. Moreover, the arc-over of high frequency 
approximates that formed on the insulators during a heavy surge on the 
line, and insulators tested successfully under this condition should make 
the line safer than insulators not so tested. In experiments with different 
makes of insulators it was found that the best porcelain would withstand 
successfully unlimited punishment at high frequency, while those porcelains 
which were porous or slightly porous would succumb quickly. These tests 
were made by first adjusting the voltage to the maximum that the 
porcelain would stand, and then closing and opening the switch a given 
Fig. 4.—-High-frequency 125,000-volt testing-set. 
number of times. Such tests are severe, and correspond perhaps to mild 
lightning. On cracking up insulators punctured in the oscillator the same 
hair-like punctures were found as in the insulators described earlier in the 
paper, indicating that the weakening of the porcelain by high-frequency 
punctures probably does actually occur on the line. 
Various methods of locating weak insulators while on the line were 
tried, with more or less success. Shunting a telephone-receiver from the 
pole to earth was tried,* but as the insulator-pins are earthed the method 
did not give reliable results and was abandoned. A fairly successful 
method, and one involving the least time and expense, was a patrol of the 
* B. G. Flaherty, Trans. Am. Inst. Elec. Eng., p. 1095, 1916. 
