The resistivity of an insulator varies greatly with the
purity and surface condition of the material, time of application of the
voltage, and in some cases with the magnitude of the applied stress. It usually
decreases rapidly as the temperature is increased (in many cases changing by
orders of magnitude) and may be reduced temporarily or permanently by
irradiation.
Conventionally, resistivity measurements are made one
minute after application of the measurement voltage. For almost all insulating
materials the current at this time is not a true conduction current, but an
absorption current involving storage of charge in the insulator. The
equilibrium conduction current may be several orders of magnitude smaller.
Some typical one-minute values for both resistivity and
surface resistivity are given below for normal temperatures and
50–60% relative
humidity. Surface resistivity is defined as the resistance between opposite
edges of a square (the size is immaterial) and the unit is the ohm (per
square).
|
Material |
|
Volume resistivity |
|
ohm metre |
|
|
Surface resistivity |
|
ohm (per square) |
|
| |
|
|
|
Beeswax (fresh surface)
. . . . |
1012–1013 |
~ 1014 |
|
Boron
. . . . . . . . . |
1010 |
|
|
Ceramics . . . . . . . . |
|
|
|
Alumina . . . . . . . . |
109–1012 |
|
|
Porcelain . . . . . . . . |
1010–1012 |
1011 glazed |
|
|
|
109 unglazed |
|
Pyrophyllite . . . . . . . |
1010–1013 |
|
|
Special
H.F . . . . . . . |
1014 |
|
|
Steatite
. . . . . . . . |
1011–1013 |
|
|
Titanates . . . . . . . . |
106–1013 |
|
|
Diamond . . . . . . . . |
1010–1011 |
|
|
Glass |
|
|
|
Soda–lime . . . . . . . |
109–1011 |
1010–1012 |
|
Borosilicate (Pyrex)
. . . . |
1012 |
|
|
Plate . . . . . . . . . |
2 × 1011 |
5 × 1010 |
|
Guttapercha . . . . . . . |
107 |
|
|
Hard rubber (Ebonite,
etc.) . . |
1013–1015 |
1010–1018 |
|
Iodine
. . . . . . . . . . . . . . . . |
1013 |
|
|
Ivory
. . . . . . . . . . . . . . . . |
106 |
109 |
|
Marble . . . . . . . . . . . . . . . |
107–109 |
109 |
|
Mica, sheet
. . . . . . . . . . . . |
1011–1015 |
1010–1013 |
|
,, moulded
. . . . . . . . . . . . |
1013 |
5 × 1013 |
|
Mineral oil
. . . . . . . . . . . . |
>1010 |
|
|
Mineral insulating oil
. . . . . . . . |
1011–1015 |
|
|
Paper
(dry) . . . . . . . . . . . . |
~1010 |
109–1010 |
|
Paraffin (kerosene,
coloured) . . . . |
1011–1012 |
|
|
Paraffin
wax . . . . . . . . . . . . |
1013–1017 |
1015 |
|
Plastics |
|
|
|
Acrylic (Perspex,
etc.) . . . . . . |
> 1013 |
>1014 |
|
Alkydes or polyester (no
filler) . . |
1012–1013 |
1013–1014 |
|
Aminos, melamines
(cellulose) . . |
>109 |
1012–1014 |
|
,,
,, (mineral)
. . |
109 |
1012–1014 |
|
Casein
. . . . . . . . . . . . |
107–108 |
1010–1011 |
|
Cellulose
acetate . . . . . . . . |
108–1011 |
1011–1012 |
|
Epoxy cast resin (no
filler) . . . |
1012–1013 |
3 × 107– > 1014 |
|
Phenolics
. . . . . . . . . . |
106–1012 |
108–1014 |
|
Polyamides
(nylon) . . . . . . . |
108–1013 |
1011–1015 |
|
P.C.T.F.E.† . . . . . . . . . . |
1016 |
1012–1013 |
|
P.E.T.‡ . . . . . . . . . . |
1015–1017 |
|
|
Polyformaldehyde . . . . . . . |
~6 × 1012 |
>2 × 1013 |
|
Polypropylene . . . . . . . . . |
1013–1015 |
>1015 |
|
Polystyrene (general
purpose) |
1015–1019 |
>1014 |
|
,,
(toughened) . . . . |
1010–1015 |
>1014 |
|
P.T.F.E.§ . . . . . . . . . . |
1015–1019 |
4 × 1012–1017 |
|
Polythene (high density)
. . . . |
1014–1015 |
1012–1017 |
|
,,
(low densiy)
. . . . |
1014–1018 |
>1014 |
|
Polyurethanes . . . . . . . . . |
109–1012 |
1013–1015 |
|
P.V.C.*
(rigid) . . . . . . . . . |
5 × 1012–1013 |
1012–1015 |
|
,,
(flexible)
. . . . . . . |
5 × 106–5 × 1012 |
>1014 |
|
Silicone
(glass) . . . . . . . . |
108–1012 |
>1011 |
|
Quartz, || optic
axis . . . . . . . . |
1012 |
|
|
,,  optic
axis . . . . . . . . |
1014 |
|
|
,,
fused . . . . . . . . . . |
>1016 |
3 × 1012 |
|
Shellac . . . . . . . . . . . . . . |
107 |
5 × 1013 |
|
Silicone,
oils . . . . . . . . . . . |
1012 |
|
|
,,
rubber . . . . . . . . . |
109 |
|
|
Slate . . . . . . . . . . . . . . |
105–106 |
107 |
|
Soil . . . . . . . . . . . . . . |
102–104 |
|
|
Sulphur . . . . . . . . . . . . . |
1014–1015 |
7 × 1015 |
|
Wood (paraffined)
. . . . . . . . |
108–1011 |
1012 |
|
Water
(distilled) . . . . . . . . . |
102–105 |
|
|
|
|
|
