289 
1918.] The N.Z. Journal of Science and Technology. 
Regarding the Portland cement used for jointing the shells together, 
and the shells of the cap and thimble, it has one peculiar property which 
has an important bearing upon its use in this connection — viz., that hydra¬ 
tion is not complete when the initial set takes place, and that the process 
of hydration goes on at a very slow rate almost indefinitely, and is accom¬ 
panied by an expansion of the cement. It is now an established fact that 
under the most favourable conditions not more than 20 per cent, of the 
cement used in making concrete is hydrated, and that the hydration of 
even this amount is a slow process.* Regarding the finished insulator, 
the special features and properties which have a bearing upon the life or 
endurance of the insulator in service are :— 
(1.) The difficulty of procuring a complete and thorough vitrification of 
the porcelain, giving rise to isolated patches of porous porcelain ; 
(2.) The unglazed surface at the cemented joint; 
(3.) The difference in expansion with temperature of the porcelain and 
the iron of the thimble or cap ; 
(4.) The continued hydration, accompanied by expansion of the cement. 
In regions where the climatic conditions are severe and the variation 
in temperature great, failure of insulators appears to result more frequently 
from a difference in expansion or contraction between the porcelain and 
iron cap of the type shown in fig. 2, which puts stresses on the porcelain 
which it cannot support. Again, a humid climate promotes the hydration 
of the cement at the joints of the insulators shown in figs. 1 and 2. As 
the cement expands with hydration, disruptive forces are brought into 
play on the porcelain which it cannot resist, and failure follows. Failures 
will, however, occur, though less frequently, with absence of extreme con¬ 
ditions of temperature and humidity ; and where it is impossible that these 
agencies can become operative the question then arises as to the cause 
of failure, should such occur. In the case of failure of insulators on the 
Lake Coleridge-Christchurch lines, it was impossible to accept the tempera¬ 
ture theory or the humidity theory without considerable qualifications. 
A careful study of the subject, accompanied by repeated experiments 
made at the Addington substation, and compared with experimental in¬ 
vestigations elsewhere, notably in the United States of America, leads to 
the conclusion that the ultimate cause of failure when every other cause 
is eliminated is to be found in the lack of complete vitrification of the 
porcelain. This condition is liable to occur in patches or spots whilst the 
rest of the bulk of the porcelain may be perfectly sound. 
Porcelain, unless thoroughly vitrified, is porous, and if such a con¬ 
dition exists at the joints of the insulator shown in fig. 1, for instance, and 
if the surface is there unglazed, as it is in most insulators, moisture will 
sooner or later penetrate the porcelain at the unvitrified spot, and this 
will inevitably cause the insulator to fail under service conditions. The 
moisture is absorbed from the cement, which, of course, is wet when first 
applied ; and, further, a dry spell of weather followed by a cold and humid 
condition of the atmosphere will cause the cement to absorb moisture, and 
this will in turn penetrate into the unvitrified portions of the porcelain 
at the joint between the cement and porcelain. A similar action takes 
place under ordinary weather conditions if the line, after being in service 
during the day, is cut out during the night. In the former condition, owing 
to the action of the current and the warmth of the atmosphere, the moisture 
is expelled from the cement, and if this is followed by a cold night and a 
humid condition of the atmosphere the cement breathes in the moisture 
and transfers it to the porous patch in the porcelain, with the result that 
when electric pressure is applied preparatory to putting the line in service 
* N. C. Johnson, Engineering Record, vol. 71, No. 11, p. 322, 1915. 
19—Science 
