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The creation of a magnetic field by a conductor in its own neighborhood when carrying a current of electricity is one of the most important phenomena of electrical science.

Electrical energy must be expended in producing a magnetic field. When a current of electricity is turned on in a wire the magnetic field grows around the wire, some of the energy of the current being used for the building process.

This reactive effect of the surrounding magnetic field is one reason why electric currents do not instantly rise to their full value.

FIG. 14. Diagram showing how Lines of Force Form about a Loop of Wire.

If a wire is connected to a battery or some other source of electric current and a portion of the circuit twisted so as to form a loop, the entire space enclosed by the loop will be a magnetic field and possess magnetic properties.

By forming a wire into a spiral coil the combined effect of each individual turn is concentrated in a small space and a powerful field of force is produced. If the coil is provided with an iron core, the lines of force can be concentrated and will exercise a very powerful attractive effect upon any neighboring masses of iron or steel. Such a coil is called an electromagnet. A hollow coil without any core is called a solenoid.