188 
The N.Z. Journal of Science and Technology. 
[Nov. 
lines at night under different weather conditions. An incipient fault can 
sometimes be detected by the unusual glow on the insulator or by the 
spitting sound of a discharge. Insulators detected in this way and removed 
were often found to be weak. Meggering the insulators on the line was 
tried, and while it was found somewhat difficult to get the wire-brush 
contacts on to the cement joints, yet the method proved fairly successful. 
On suspension units the megger test is not so difficult to carry out. 
The writers wish to recognize the valuable work done by Dr. Coleridge 
Farr in connection with the hydrostatic-pressure tests, and by Mr. H. E. 
Philpott, who did most of the actual experimenting in connection with 
these investigations. 
Conclusions. 
The length of service of an insulator either of the pin or suspension 
type is partly a question of design, but depends mainly on the care with 
which the pieces are manufactured and fired, the firing of the wire being 
apparently the most important operation. 
The latest designs of pin-type insulators are much superior mechanically 
and electrically to the older designs, and, provided the ware is thoroughly 
vitrified, much less trouble should be experienced in future installations 
of pin-type insulators than has occurred in the past. 
Temperature stresses cause many failures, and this problem appears 
the most difficult to overcome. Improvement in the design of the insu¬ 
lators will no doubt reduce these troubles in the future. 
The installation of protective apparatus to reduce the effect of transient 
high voltages would minimize the risk of puncture of the porcelain. 
It is important that the general condition of the line-insulation be 
determined periodically by the testing of batches of insulators either on 
the line or in the test-room, after removal from the line, so that a systematic 
weeding-out of impaired units may be carried out, and interruptions to 
service thus reduced. 
It is anticipated that as a result of the improvement in the quality of 
the material used and the methods of manufacture, in which matters keen 
research is being carried on continuously both in America and in England, 
it will be possible to supply porcelain so thoroughly vitrified that it is not 
liable to deterioration in the direction described, and that it will then be 
possible to reduce substantially the size and cost of insulators giving a 
still higher degree of reliability than has been available in the past. 
The matter is well summarized in a short article by F. W. Peek in the 
Electrical World (New York) for 24th January last, page 209, in which 
he states : 
The apparent deterioration of the porcelain is for the greater part due to one 
or the other of the following causes : (1) Gradual mechanical cracking due to the 
expansion of cement or tight-fitting metal parts, to internal strains caused by firing, 
or to brittle porcelain ; (2) gradual absorption of moisture due to porous porcelain. 
The solution seems to be to start with an insulator designed to avoid any 
possible cracking of the porcelain parts from expansion of cement or metal. Years 
of service have so far been the best criterion of design. The material should then be 
selected of uniform quality, reasonable toughness, and freedom from porosity and firing- 
strains. The electrical strength of the completed unit should, of course, be reasonable; 
but this is really secondary to the other characteristics, since the highest electrical 
strength vanishes when a crack appears. 
Electrical, mechanical, and porosity uniformity tests are necessary. Extremely 
accurate tests are not important, but it is important to have tests that can be very 
quickly made, in order to be able to find at once any trouble due to firing or poor 
materials. 
