Vive La Resistance

Resistance, in particular the kind that is in opposition to motion, may or may not be beneficial.  All motion resistance involves one object's atoms bumping against another object's atoms and exchanging kinetic energy, which on a microscopic level is heat.
Resistance related to fluids rubbing against a solid object is an important area of study in the mechanical engineering field of "fluid dynamics", where examples can range from the resistance of water flowing in pipes, to the air drag on a moving car, or a space shuttle suffering blazing re-entry into the atmosphere...or even plaque in a coronary artery slowing the flow of blood.  The narrowing of an artery causing resistance also has an analog in vehicular traffic flow.  The GW Bridge is famous for cars slowing their pace due to the narrowing of traffic lanes (construction, breakdowns, toll booths, etc.) causing some drivers to gain a form of kinetic energy (getting hot under the collar).

Heat conducts through solids by the transfer of vibrational kinetic energy from atom to atom.  How easily the heat current, Q (watts), conducts through a solid due to an applied temperature difference, ΔΤ, is described by Fourier's law, Q = ΔΤ / RTH , where RTH is the thermal resistance of the solid.  Think of the temperature difference as the driving force and the heat current, Q, as the resulting flow.

Electricity conducts through solids by the flow of unbound electrons.  Electrical resistance is due to the fixed atoms or molecules getting in the way of the flow of electrons.  A voltage source acts as a force or pressure to drive those electrons through the solid from one end to the other.  As the electrons bounce off the atoms they impart some kinetic energy causing the fixed atoms to vibrate more, elevating the temperature of the material.  Besides the type of atoms, the geometry of the solid also affects its electrical resistance.   In 1827 Georg Ohm discovered the linear relationship I = ΔV/R , where ΔV, the voltage difference across the resistor, is like electrical force or pressure, R is resistance in ohms, and I is the resulting current in amperes.   It's the electrical analog to Fourier's heat law.     

Electrical resistance is so useful in electrical engineering that "resistors" are manufactured to have particular values of resistance.  The simplest electrical resistor is the ubiquitous, fixed resistor put into electronic circuits to control/limit current flow.  Those unheralded resistors have constant resistance values (in "ohms") and, as with all resistors, are "passive" in the sense that they have no energy source of their own.  Some fixed resistors are made to take advantage of the exchange of heat from the electrons to the atoms.  Examples are electrical heaters and incandescent light bulbs.       

Slightly more complex resistors are those whose resistance can be changed by turning a knob or sliding a lever.  They are "potentiometers", where controlling a resistance manually may be used, for example, to adjust the volume of a radio.

The virtuosos of all the resistors are the ones which respond to outside parameters, changing their resistance to reflect the external conditions.  They are known as "transducers", transmogrifying some physical parameter into an electrical signal which can easily be detected, filtered, amplified, stored, displayed on a computer screen, etc.  All fields of engineering use them to convert from a physical parameter to an electrical signal.   The remainder of this article briefly describes some of the many sensitivities of resistors to physical inputs.

Some of the earliest electrical transducers used the sensitivity of a solid's electrical resistance to temperature.  These electrical thermometers are called "thermistors".   Today, many cars use a heated platinum wire's resistance change, when cooled by air rushing past it to act as an "air mass meter", for measuring air flow so as to regulate fuel mixture.
An early engineering application of resistance changing with pressure is the carbon microphone used to transduce sound waves.  An increase in sound pressure makes the carbon denser and lowers its electrical resistance.

Simple light meters can be made using a semiconductor as the material because its resistance can be very sensitive to light.  

An example of a geometric transducer is a rotary sliding-contact potentiometer, whose resistance can be used to monitor the throttle plate's angular position in a car's engine.  

If a small change in length is to be measured, a thin flexible metallic resistor can be used.  As the metal is deformed (strained), its atomic spacing changes and consequently so does its electrical resistance.  Such a motion transducer is called a "strain gauge" with uses ranging from measuring deformation in a bending beam to the expansion of land over a slowly awakening volcano.   

The salinity of water can be measured using the water's resistance.  The water content of lumber is measured using the resistance of the wood.  A bathroom water leak can be detected by a special resistor on the floor.

Newer applications of electrical resistance's sensitivity to chemical stimulants are evolving using exotic solids.   An electronic "nose on a chip" uses the resistive sensitivity of a particular solid to detect a particular odor.  Pathogen sensors chips are being developed using single strands of DNA on a resistive substrate.  One being tested uses DNA, which must be kept alive and nourished, and can only bond with a certain bio-agent gene.  Once the DNA binds, a process occurs changing the resistance of a sensor below the DNA.    

Some modern nonvolatile magnetic memories use the "giant magnetoresistive" effect, where a magnetic field produces a change in the resistance of a ¬nanometers-thick conductive layer.  The devices make good magnetic switches that are so reliable some implanted defibrillators now incorporate them, and the effect is commonly used by hard-drive read heads.

Information in today's ubiquitous flash drives, and some other RAM memories, is read by measuring the resistance of the transistor(s) holding the digital memory bit(s).

In some sense, our subject the resistor can even claim parentage to the transistor, whose name comes from "transfer resistance".  Transistors act like controlled resistors, and they "transfer" resistance values from one end of the transistor to another.   But that's another story, and I have to go suffer the resistance of the GW Bridge at rush hour.