Ohms x Amperes = Volts, or R x I = E
That is, if you multiply the resistance of the circuit in ohms by the current in amperes the result will give you the electromotive force in volts.
From this you will see that if you know the value of any two of the constants you can find the value of the unknown constant by a simple arithmetical process. This relation between these three constants is known as Ohm's Law and as they are very important you should memorize them.
What the Watt and Kilowatt Are.--Just as horsepower or H.P., is the unit of work that steam has done or can do, so the watt is the unit of work that an electric current has done or can do. To find the watts a current develops you need only to multiply the amperes by the volts. There are 746 watts to 1 horsepower, and 1,000 watts are equal to 1 kilowatt.
Electromagnetic Induction.--To show that a current of electricity sets up a magnetic field around it you have only to hold a compass over a wire whose ends are connected with a battery when the needle will swing at right angles to the length of the wire. By winding an insulated wire into a coil and connecting the ends of the latter with a battery you will find, if you test it with a compass, that the coil is magnetic.
This is due to the fact that the energy of an electric current flowing in the wire is partly changed into magnetic lines of force which rotate at right angles about it as shown at A in Fig. 32. The magnetic field produced by the current flowing in the coil is precisely the same as that set up by a permanent steel magnet. Conversely, when a magnetic line of force is set up a part of its energy goes to make up electric currents which whirl about in a like manner, as shown at B.
Self-induction or Inductance.--When a current is made to flow in a coil of wire the magnetic lines of force produced are concentrated, as at C, just as a lens concentrates rays of light, and this forms an intense magnetic field, as it is called. Now if a bar of soft iron is brought close to one end of the coil of wire, or, better still, if it is pushed into the coil, it will be magnetized by electromagnetic induction, see D, and it will remain a magnet until the current is cut off.
Mutual Induction.--When two loops of wire, or better, two coils of wire, are placed close together the electromagnetic induction between them is reactive, that is, when a current is made to flow through one of the coils closed magnetic lines of force are set up and when these cut the other loop or turns of wire of the other coil, they in turn produce electric currents in it.