Resistance and the Ohm.--Just as a water pipe offers a certain amount of resistance to the flow of water through it, so a circuit opposes the flow of electricity in it and this is called resistance. Further, in the same way that a small pipe will not allow a large amount of water to flow through it, so, too, a thin wire limits the flow of the current in it.
If you connect a resistance coil in a circuit it acts in the same way as partly closing the valve in a pipe, as shown at A and B in Fig. 31. The resistance of a circuit is measured by a unit called the ohm, and it is expressed by the symbol R. A No. 10, Brown and Sharpe gauge soft copper wire, 1,000 feet long, has a resistance of about 1 ohm. To measure the resistance of a circuit an apparatus called a resistance bridge is used. The resistance of a circuit can, however, be easily calculated, as the following shows.
What Ohm's Law Is.--If, now, (1) you know what the current flowing in a circuit is in amperes, and the electromotive force, or pressure, is in volts, you can then easily find what the resistance is in ohms of the circuit in which the current is flowing by this formula:
Volts E
--------- = Ohms, or --- = R
Amperes I
That is, if you divide the current in amperes by the electromotive force in volts the quotient will give you the resistance in ohms.
Or (2) if you know what the electromotive force of the current is in volts and the resistance of the circuit is in ohms then you can find what the current flowing in the circuit is in amperes, thus:
Volts E
----- = Amperes, or --- = I
Ohms R
That is, by dividing the resistance of the circuit in ohms, by the electromotive force of the current you will get the amperes flowing in the circuit.
Finally (3) if you know what the resistance of the circuit is in ohms and the current is in amperes then you can find what the electromotive force is in volts since: