1919.] 
Departmental Reports. 
133 
“ Insulation of line only per phase varied with weather conditions between 
0 3 and 17 megohms. After a night of misty or damp weather in the 
plains it would fall to 0*7 megohm, and then come up through the day to 
2 or 6 megohms.- After continued north-westerly weather the insulation 
resistance reached 17 megohms.” The line referred to upon which the 
measurements were made is the south line between Lake Coleridge and 
Christchurch, extending over a length of 61-3 miles. 
Reduced to the mile basis the measurements ranged from 48*9 megohms 
to 2,771 megohms per mile per phase, and the inverse of this, the leakage 
conductance, from 0-02 x 10 -6 to 0-00036 x 10 -6 units per mile per 
phase ; a normal-day value appears to be about 2 megohms per phase 
per circuit, which is 122-6 megohms per mile per phase per circuit, or a 
leakage conductance of 0-00815 x 10~ 6 units per mile per phase. The 
insulators are of the pin type, and a value of 0-008 x 10~ 6 may be taken 
as the normal leakage conductance per mile per phase for pin insulators. 
The insulation resistance of suspension insulators would be somewhat 
higher than this, and the value of 0 005 x 10 -6 is herein adopted as 
the normal-day leakage conductance per mile per phase of a line with 
suspension insulators. 
Capacity, Capacitance, Permittance. 
These are interchangeable terms for what used to be called simply 
“ capacity,” meaning the extent to which an electric system yields to 
electrostatic stress ; the greater the strain for a given stress the greater the 
capacity; the less its power of resistance the greater the capacity. The 
term, to say the least, is not at all helpful, and the word “ permittance ” 
was proposed by Heavyside in place of it. It is denoted herein by the 
symbol S, and its value in microfarads per mile of single wire of a 
symmetrical three-phase system corresponding to different values of d/a 
is given in D 73. The symbols d and a have the same significance as 
before. The values are calculated from the formula 
1 
S = 
a 
C.G.S. electrostatic units per centimetre. 
1 _ 
d x 30-48 x 5280 C.G.S. electrostatic units per mile. 
a 
__ x ^ ^ X C.G.S. electromagnetic units per mile. 
2 log- 9 x 10 20 ° F 
a 
1 
- - A 30-48 x 5280 x 10° 
~ 2 log- X - 9 x IQ 2 Q -farads per mile. 
° a 
— 2 ^g- X o- 59 i microfarads per mile. 
a 
The formula is an approximation to the capacity of three symmetrically 
spaced and isolated wires, and is further influenced by the earth, and to 
some extent by other wires in the vicinity, and will vary amongst them¬ 
selves according to their position relatively to the earth. These correc¬ 
tions, whilst not negligible, are yet smaller than the variation of capacity 
