292 
The N.Z. Journal of Science and Technology. 
[Sept, 
in diameter which penetrates all four shells. In the insulator on the left 
the path of the current is not so direct, with the result that the several 
shells have been shattered. The manner in which an insulator fails is a 
process of considerable interest. It must not be assumed that an insulator 
succumbs to the first attack. The process is really a slow one, beginning 
with a hair-like puncture of some one or other of the shells as the result 
of an abnormal voltage, which, however, is not sufficient to cause a 
complete failure. On a subsequent occasion when an abnormal vokage 
occurs, possibly as the result of the failure of an insulator in some part of 
the circuit, the insulator which suffered injury on a former occasion is now 
weaker to resist the assault and suffers further injury ; possibly a second 
shell is damaged by hair-like punctures, which are, however, sealed up. 
The next assault or application of abnormal voltage succeeds in cracking 
Fig. 3. —-Damaged insulators, Lake Coleridge-Christchurch transmission-line. 
one or more insulators, due to the generation of heat, and the insulator 
is now ripe for a final breakdown. The first wet day with a wind in a 
direction which is favourable for driving the moisture into the cracked 
porcelain may cause the insulator to fail under normal service conditions. 
A surge or fluctuation of voltage is now set up which may be the beginning 
of a long series of attacks upon another insulator with a hidden defect, 
sufficient in course of time to lead to complete failure. 
Since the above article was written an important contribution to the 
subject of the design and manufacture of porcelain insulators has appeared 
in the Proceedings of the American Institute of Electrical Engineers A It 
contains an improved design of insulator based upon a theoretical and 
experimental study of the subject combined with a knowledge of the 
ceramic art. 
* G. I. Gilchrest, Application of Theory and Practice to the Design of Trans¬ 
mission-line Insulators, Proc. Am. Inst. Elec. Eng., vol. 37, No. 6, p. 571. 
