1920.] Birrs and Ferguson.—High-tension Insulators. 185 
1919, pp. 302-7. For the purpose of these experiments a hole 3 in. in 
diameter and 6J in. deep was bored in a solid steel block 5 in. square by 
9 in. deep. A steel cover 1 in. thick was fitted to the block and fastened 
down by eight f in. bolts. Specimens of porcelain were placed in the 
cavity and fuchsin solution was poured in ; red ink was also used and found 
to answer as well as the fuchsin. The cover was then bolted on and the 
vessel connected to an air-pump capable of producing a pressure of 2,000 lb. 
per square inch. Under such a pressure it was expected that the coloured 
liquid would be forced into the porcelain where it was at all porous, the 
extent of penetration being made visible on breaking the specimens. 
Test No. 1 was carried out by placing in the pot ten pieces of stock 
66,000-volt insulator porcelain broken from various parts of the insulator, 
some pieces glazed and others unglazed. After being subjected to an 
average pressure of 2,000 lb. per square inch for fifty hours the pieces were 
taken out and broken, and in each case there was no sign of penetration. 
Test No. 2 was similar to the above and on the same class and make 
of porcelain. Of the ten specimens, two showed slight penetration. From 
the puncture tests and flash-over tests on this class of porcelain it appears 
that the porcelain was slightly porous, and the dull appearance of the 
fracture supported this belief. 
Test No. 3 was made on nine pieces broken from a 50,000-volt insulator. 
Test pressure was the same as for tests 1 and 2. None of these specimens 
showed any trace of penetration, a result which was expected from the 
behaviour of this class of insulator under puncture and flash-over tests, 
the silky fracture and close grain of this porcelain suggesting complete 
vitrification and absence of porosity. 
Test No. 4 : Ten pieces were broken from parts of three insulators 
which had been removed under suspicion from the transmission-lines after 
three years’ service, and put under 2,000 lb. pressure per square inch for 
sixty-nine hours. As the fuchsin colouring photographs poorly, the coloured 
area was marked with pencil. The specimens were selected from different 
parts of each insulator, and from the chalky nature of the fractures it was 
suspected that the pieces were porous. The results are given in the tables 
and photograph accompanying the article referred to (pp. 305-6). It will 
be noticed that the photographs disclose a porous core between the outer 
glazed surfaces, while the porcelain adjacent to the glazing is, of course, 
more completely vitrified. 
In all of the experiments on absorption it was found exceedingly 
difficult to force liquid into porcelain even when other tests indicated 
fairly definitely that the porcelain was porous to a degree.* 
A number of puncture tests under oil were carried out on pieces of insu¬ 
lators removed from the line, as well as on pieces of stock insulators of the 
same make as those in service but better vitrified. The puncture tests 
were interesting as showing the variation in dielectric strength of the 
porcelain in service, some of which withstood as high as 90,000 volts for 
pieces in. in thickness. The stock replacement insulators, which, as 
previously noted, were of the same make as those installed on the lines, 
had been stacked in their crates in the open for about three years. The 
puncture voltage of pieces broken from these insulators averaged about 
85,000 volts, several specimens holding up at 100,000 volts. In only one or 
two instances the English porcelain from 50,000-volt insulators punctured 
below 100,000 volts, the limit of the testing equipment. 
* J. C. Clark, Trans. Am. Inst. Elec. Eng., p. 1453, 1916. 
