290 The N.Z. Journal of Science and Technology. [Sept. 
such insulators fail either partially or completely. Thus the cement acts 
detrimentally in two ways : it expands by continued hydration, and by 
exerting pressure in the porcelain breaks it mechanically ; and it also acts 
as a conductor of moisture from the atmosphere into such parts of the 
insulator in contact with it as may be in a porous condition, which results 
in failure for electrical reasons. Temperature variation also acts in two 
ways : if extreme it breaks the porcelain by unequal expansion of the 
porcelain and the metal cap or thimble, and if not extreme it still promotes 
absorption of moisture in the presence of the ordinary amount of humidity. 
In the case of the Lake Coleridge insulators, where extreme conditions 
of climate do not prevail, it did not seem at all probable that the insulators 
failed by unequal expansion ; neither was there any evidence of the 
expansion of the cement by hydration ; and, in any case, it was not con¬ 
sidered that the insulators had been long enough in use to give effect to 
this cause. Finally, after observation in the field and experiment in the 
laboratory, combined with a study of the literature on the subject, the 
conclusion was reached, as already stated, that porosity in the porcelain 
was the prime cause of failure. 
The process by which failure takes place is easily explained. Briefly, 
the history of the average insulator-failure is as follows : There is always 
present in the atmosphere a number of free electrons, the normal density 
being 16,000 per cubic centimetre. This may appear a large number, 
but when compared with the number of molecules in the air at normal 
temperature and pressure — viz., 27 X 10 1 *— the number is infinitesimal. 
The presence of free electrons renders the atmosphere conducting, for 
electrical conductivity in any material is determined by the number of 
free electrons in it. The number mentioned is not sufficient, however, 
to render the atmosphere conducting to any appreciable extent, and for 
practical purposes air under normal conditions is regarded as the best 
insulating-material in existence. Air is, however, defective in strength 
to resist certain influences which tend to increase ionization to such an 
extent as to render it conducting. 
Ionization of the air is promoted by disruptive collision between a free 
electron and a molecule of air, but although frequent collisions take place 
the energy is not sufficient to liberate another electron from the atom. The 
mean free path of an electron at ordinary temperature and pressure is 
6 X 10 -5 cm., but this length of path is not sufficient under normal con¬ 
ditions to enable the electron to acquire sufficient energy to ionize by 
collision. If, however, the air be subject to an electrostatic stress of 
29,800 volts per centimetre or more, enough energy is imparted to the 
moving electron to detach another electron by collision with a molecule 
of air, the cumulative effect of which renders the air a conductor of 
electricity. 
By referring to the pin insulator (fig. 1) it will be readily understood 
that there is a region of high electrostatic stress between the wire or clamp 
and the metal thimble into which the bolt is screwed. Linder certain 
working-conditions any air-cavities in that portion of the porcelain or the 
cement that is under the influence of high electrostatic stress will be ' 
subject to ionization, causing local heating in the cement or porcelain, 
which tends to crack the porcelain. Many insulators, no doubt, fail in 
this way ; but it is seldom that cavities exist in the porcelain, and the 
failures from this cause would not be large. Cavities are, however, always 
present in the cement, and it is possible that a number of failures occur 
owing to local heating of the cement and the transfer of the heat to the 
porcelain. This agency can become operative, therefore, in a perfectly 
dry climate without any extremes of temperature, and is due simply to a 
combination of the presence of cavities and of high electrostatic stress as 
