Just as there is no such thing as a perfectly resistance-free substance, there isn’t a truly infinite
resistance, either. Even air conducts to some extent, although the effect is usually so small that it can
be ignored. In some electronic applications, materials are selected on the basis of how “nearly infi-
nite” their resistance is.
In electronics, the resistance of a component often varies, depending on the conditions under
which it is operated. A transistor, for example, might have high resistance some of the time, and low
resistance at other times. High/low resistance variations can be made to take place thousands, mil-
lions, or billions of times each second. In this way, oscillators, amplifiers, and digital devices func-
tion in radio receivers and transmitters, telephone networks, digital computers, and satellite links
(to name just a few applications).
Conductance and the Siemens
Electricians and electrical engineers sometimes talk about the conductance of a material, rather than
about its resistance. The standard unit of conductance is the siemens, abbreviated S. When a com-
ponent has a conductance of 1 S, its resistance is 1 Ω. If the resistance is doubled, the conductance
is cut in half, and vice versa. Therefore, conductance is the reciprocal of resistance.
If you know the resistance of a component or circuit in ohms, you can get the conductance in
siemens: divide 1 by the resistance. If you know the conductance in siemens, you can get the resist-
ance: divide 1 by the conductance. Resistance, as a variable quantity, is denoted by an italicized, up-
percase letter R. Conductance, as a variable quantity, is denoted as an italicized, uppercase letter G. If
we express R in ohms and G in siemens, then the following two equations describe their relationship:
G = 1/R
R = 1/G
Units of conductance much smaller than the siemens are often used. A resistance of 1 kΩ is
equal to 1 millisiemens (1 mS). If the resistance is 1 MΩ, the conductance is one microsiemens (1 µS).
You’ll sometimes hear about kilosiemens (kS) or megasiemens (MS), representing resistances of 0.001
Ω and 0.000001 Ω (a thousandth of an ohm and a millionth of an ohm, respectively). Short lengths
of heavy wire have conductance values in the range of kilosiemens. Heavy metal rods can have con-
ductances in the megasiemens range.
Suppose a component has a resistance of 50 Ω. Then its conductance, in siemens, is 1/50 S,
which is equal to 0.02 S. We can call this 20 mS. Or imagine a piece of wire with a conductance
of 20 S. Its resistance is 1/20 Ω, which is equal to 0.05 Ω. You will not often hear the term mil-
liohm. But you could say that this wire has a resistance of 50 mΩ, and you would be technically
right.
Determining conductivity is tricky. If wire has a resistivity of 10 Ω/km, you can’t say that it has
a conductivity of 1/10, or 0.1, S/km. It is true that a kilometer of such wire has a conductance of
0.1 S, but 2 km of the wire has a resistance of 20 Ω (because there is twice as much wire). That is
not twice the conductance, but half. If you say that the conductivity of the wire is 0.1 S/km, then
you might be tempted to say that 2 km of the wire has 0.2 S of conductance. That would be a mis-
take! Conductance decreases with increasing wire length.
Figure 2-5 illustrates the resistance and conductance values for various lengths of wire having a
resistivity of 10 Ω/km.
22 Electrical Units