Static Electricity 2000 Edition

D.1.43 Ohms per Square.
A unit of measure used to describe
surface resistivity that reflects the resistance value between two
electrodes that form two sides of a square and is independent
of the size of the square. The resulting value indicates how eas-
ily electrons can flow across a surface. It is normally used as a
resistivity measurement of a thin conductive layer or material
over a relatively insulative base material.
D.1.44 Propagating Brush Discharge.
An energetic discharge
caused by electrical breakdown of the dielectric layer in a capac-
itor. The capacitor is typically formed by charged plastic coating
on a metal substrate, although plastic pipe and tote bins can
also form the required charged double layer. The effective
energy can exceed 1000 mJ, causing both shock to personnel
and ignition hazards for a wide variety of materials, including
dusts in air.
D.1.45 Relaxation Time Constant (
ττττ). The time, in seconds,
for charge to decay by Ohm’s law to e
−1
(36.7 percent) of its ini-
tial value. For a capacitor, the relaxation time constant is the
product of resistance (ohms) and capacitance (farads).
D.1.46 Resistance (R).
The opposition that a device or mate-
rial offers to the flow of direct current, equal to the voltage
Table D.1.22 Dielectric Properties of Selected Materials
Material
Dielectric 
Constant
Dielectric Strength 
(V/m)
Bakelite
4.9
2.4 
× 10
7
Cellulose acetate
3.8
1.0 
× 10
7
Mica
5.4
1.0 
× 10
8
Plexiglas, Lucite
3.4
4.0 
× 10
7
Polystyrene
2.5
2.4 
× 10
7
Porcelain
7
6.0 
× 10
6
Titanium dioxide
90
6.0 
× 10
6
Barium titanate
1200
5.0 
× 10
6

---

77–
54
STATIC ELECTRICITY
2000 Edition
drop across the element divided by the current through the
element; also known as electrical resistance.