# How is resistance measured?

Mikhail Voronchuk
January 9, 2013

Immediately make a reservation that we will discuss the measurement of resistance to electric current. What is it, and how is resistance measured?

## Three whales

Where does such resistance come from? All materials in nature, in terms of electrical conductivity, are divided into 3 categories - insulators, semiconductors, and conductors. The former do not conduct electrical current at all (for example, glass, plastic, air), the latter do not transmit current only under certain conditions (silicon, germanium), and all modern electronics are built on their basis. But we are interested in the last - all familiar guides. Ordinary copper wire, wire that connects your computer to a power outlet - all these are conductors.

So how can conductors resist electric current? The fact is that there is no perfect conductor in nature. In any, even the "purest" conductor, there are always some of the impurities that resist electrons moving in the body of the conductor.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 the basis of the effect of heating elements and fuses).

## A bit of math

What is the measured resistance of the conductor, or rather, the electric circuit? The unit of measurement of this quantity is named after the physicist Georg Simon Ohm. Yes, the same Ohm whose Law we all taught in school. In the technical literature is denoted by the letter "omega". The resistance itself in the calculations is written as "R" (U is voltage, I is current, P is power, etc.). What does this quantity mean? Consider an example. According to the law of the same Ohm, if our conductor has a resistance of 1 Ohm, applying a voltage of 220 volts to its ends, we get a current (current = voltage divided by resistance) of 220 amps. Multiplying the current by voltage, we will know the power: 220 volts * 220 amps = 48,400 watts, or 48 kilowatts. This is a VERY high power that no household wiring can handle. In fact, this current will be a short circuit current. This shows how important it is to know the exact resistance of the circuit, before applying voltage! Fortunately, it is not so difficult to recognize him, and even it is not necessary to carry out any calculations.There are special measuring devices - ohmmeters, which show the value of resistance to direct current. Their kind of megohmmeters - designed to measure large values ​​of resistance, and are used mainly to test the insulation. Now to find ohmmeters as separate devices is difficult. Most of them are part of the combined instruments - avometers, or multimeters, which are sold in each stall Chinese goods.

So, good measurements to you!