1918.] The N.Z. Journal of Science and Technology. 291 
the result of a surge on the lines. The question of the distribution of 
electrostatic stress is an important one in this connection. It must be 
conceded that a porcelain insulator of the type in use to-day is faulty 
when regarded from an electrical standpoint, as it tends to promote a 
concentration of electrostatic lines, with the result that although the 
average density may be within safe limits even under extraordinary voltage 
conditions such as result from a surge of voltage on the line, yet the 
distribution of density may be such as to enable the density to exceed the 
safe limit at some point or other, a condition which is aggravated by 
cavities and by lack of vitrification of the porcelain in the path of the 
electrostatic lines. 
Upon regarding the design of a high-tension insulator, as, for instance, 
that depicted in fig. 1, it will be seen that it favours a concentration of 
stress at the thimble, in proof of which may be adduced the fact that a 
failure of the inner shell is more frequent than any other. Insulators have 
been designed which satisfy the electrical requirements, notably by Randal,* 
but there seems to be some insuperable difficulty in manufacturing an 
insulator so as to satisfy ideally electrical conditions, and we are perforce 
compelled to accept a compromise between the possible and the ideal. In 
spite of this, however, a high-tension insulator of present-day design may 
be regarded as perfectly reliable provided only that the porcelain is 
thoroughly and completely vitrified in all its parts ; and, as a further 
precaution in the event of a defect in the porcelain occurring in the critical 
range between the clamp and the thimble, the following conditions should 
be observed :— 
(1.) The interface of the joints should be glazed, as is the surface of 
the insulator generally. 
(2.) Some non-porous and non-expanding substance should be employed 
for jointing purposes, and if Portland cement be used for this 
purpose the exposed faces should be coated with some water¬ 
proof varnish. 
(3.) The face of the joint between the shells should be as small as 
possible, and set horizontally, with a recess above it, so as to 
prevent water being forced against the cement face by wind- 
pressure, as in the present designs. 
Some good results have been obtained in this way on the Lake Coleridge - 
Christchurch lines, where the practice has been adopted of thoroughly 
drying out an insulator and of varnishing the cement surface before the 
insulator is put in service, and so far no insulator thus treated has failed. 
A question which occurs to one on regarding the present-day designs 
in the light of experience is whether the prevailing form would have been 
evolved if the porcelain had been thoroughly and uniformly vitrified as a 
general rule. It is now known that the porcelain used on high-tension 
transmission-lines was defective from the first, and that it was rather an 
exception than the rule for the whole of the shells to be thoroughly 
vitrified throughout. Such defects, if in the path of high electrostatic 
stress, would cause ionization within the porcelain, which would tend to 
expand and to appear as a glow on the outside. This in turn would induce 
leakage-paths over the porcelain, to be remedied by extending the shells, 
and so give rise to the present-day form. It is a question for manufacturers 
to consider whether it is not possible to produce a simpler and perhaps 
cheaper form of high-tension pin insulator, given good sound porcelain 
and a mind free from the influence of present-day designs. 
Fig. 3 is a photograph of two insulators which have failed in service 
by being punctured. The insulator on the right shows a hole some § in. 
* K. C. Randal, Proc. Am. Inst. Elec. Eng., vol. 33, p. 1215. 
