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Page 7

| Figure 1-5 Magnetic field about a coil. |

| The strength of the magnetic field inside a coil depends on the strength of the current flowing and the number of turns. Its strength is, therefore, expressed in ampere-turns, that is, amperes multiplied by the numbers of turns. Thus, a single turn carrying a very large current may produce the same effect as a great many turns carrying a small current. |

| A coil with an air core, however, produces a comparatively weak field. The strength is enormously increased by putting in a core of iron. (Fig. 1-6.) This is generally referred to as an electromagnet. |

| Figure 1-6 Coil with iron core. |

| Electromagnetic Induction.. |

| One relation between electricity and magnetism has already been demonstrated, that of producing magnetism with the aid of electric current. There is another important relation and that is the production of electricity with the aid of magnetism. |

| When a conductor is moved through a magnetic field an electrical pressureor voltageis produced in the conductor. If the strength of the magnetic field is increased (say, by replacing a simple magnet with a more powerful electromagnet), it will be found that a greater electrical pressure is now induced in the conductor cutting the lines of force. Similarly, the greater the length of the conductor, the more voltage is produced because more lines of force are cut. Also, if the speed at which the conductor is moved through |

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