What is the resistance measured ?
Outset that will focus on the measurement of resistance to electric current.What it is, and what is measured by the resistance?
Where did this resistance is taken?All materials in nature, from the viewpoint of electrical conductivity, are divided into three categories - insulators, semiconductors, and conductors.The first does not conduct electricity at all (eg, glass, plastic, air), the second - current is passed only under certain conditions (silicon, germanium), and, based on all modern electronics built.But we are interested in the past - all familiar conductors.Ordinary copper wire, wire, which is connected your computer into an electrical outlet - it all conductors.
How conductors may provide resistance to electric current?The fact that a perfect conductor does not exist in nature.In any, even the most "pure" Explorer, there is always some of the impurities that have a resistance to electrons moving in the body of the conductor.The collision of electrons with these impurities causes heating, and sometimes (if the flux density is too high, that is too much current) and the destruction of the conductor (this is based on the action of the heating elements and fuses).
What is measured conductor resistance, or more precisely - the circuit?The unit of measure of this magnitude, is named after the physicist Georg Simon Ohm.Yes, the same Ohm, whose law we all learned in school.denoted by "omega" letter in the technical literature.Needless resistance calculations are reported as "R" (U - voltage, I - current, P - power, etc.).What does it mean And this magnitude?Consider an example.According to the law, the same Ohm if our conductor has a resistance of 1 ohm, attaching to its ends voltage of 220 volts, we obtain a current (current = voltage divided by resistance) of 220 amps.Multiplying the current to voltage, we find power: 220 * 220 = 48400 watt amps, or 48 kilowatts.This is a big power, which will not sustain any household wiring.In fact, this current will short-circuit current.This shows how important it is to know exactly resistance of the circuit, before energizing!Fortunately, I learn it is not so difficult, and do not even need to carry out any calculations.There are special measuring instruments - ohm meters, which show the value of resistance to direct current.Their kind Insulation - designed to measure large values of resistance, and are used primarily for insulation testing.Now meet ohmmeters as separate devices difficult.Most of them are part of the combining devices - avometrov or multimeters, are sold in every stall Chinese goods.
Thus, successful measurement you!