288 
The N.Z. Journal of Science and Technology. 
[Sept. 
necessity. The design of the unit varies somewhat, but the commonest 
form employed is that shown in fig. 2, which is a single-shell insulator, 
six of which are arranged in series for use on a 110,000-volt circuit. Each 
insulator is fitted with a metal 
cap, which is secured to the 
porcelain by means of Portland 
cement. 
These insulators, as is well 
known, are made of porcelain, 
and a brief summary of the 
properties of this material is 
now submitted. The principal 
ingredient of porcelain is kaolin, 
a hydrated aluminium silicate 
(Al 2 0 3 .2Si0 2 -f- 2H 2 0), pro¬ 
duced by the decomposition of 
feldspar. The composition of 
insulator porcelain* as made in 
the United States of America is 
said to be 50 per cent, of kaolin 
and ball clay, 30 per cent, of feld¬ 
spar, and 20 per cent, of silica, and 
for this purpose potassium feld¬ 
spar (K 2 0.A1 2 0 3 .6Si0 2 ) is pre¬ 
ferred. The feldspar is used as a 
flux to give a dense, vitrified body 
at a temperature readily attain¬ 
able, and silica is added in order 
to extend the range of the tem¬ 
perature of vitrification and to 
prevent distortion. Porcelain as 
manufactured for insulators has 
a specific resistance of 2 X 10 12 
megohms, dielectric constant 5*3, 
ultimate limit of electrostatic 
stress 96,000 volts (R.M.S.) per 
centimetre for a thickness of 
1 cm., coefficient of linear ex¬ 
pansion with temperature 
0-0000045 C. to 0-0000065 C., 
compressive strength 30 tons per 
square inch, tensile strength 3 tons 
per square inch, and vitrifying 
temperature between 1,250° C. 
and 1,300° C.f Although porce¬ 
lain is an excellent dielectric 
at ordinary temperatures, its dielectric strength begins to fall at 100° C., 
and at 300° C. is very poor indeed, whilst it becomes fairly conducting 
when raised to a red heat. 
Fig. 2.—Suspension insulator, 
110,000 volts. 
Buller type, 
* Metallurgical and Chemical Engineering, vol. 16, No. 10, p. 589, 1917. 
fl. Lustgarten, High-tension Porcelain Line Insulators, Jour. Inst. Elec. Eng., 
vol. 49, No. 214, p. 235, 1912. 
