ELECTROMAGNETS AND GENERATORS

If an iron rod (not shown in the diagram) is poked vertically down through the coil, the fields can go through that iron much easier than through air or vacuum. 

Of course, an iron core plus a coil of wire can be an electromagnet. When the current is first turned on, the field suddenly expands, from zero to whatever the final size will be at the maximum current. 

If there are two coils, both stationary, as in Fig. 10.2, and an electric current is suddenly turned on in the left-hand coil, the magnetic field lines will quickly expand from zero to the final size, and they will get cut very fast by the stationary wires of the right-hand coil. Thus a high voltage will be generated in that coil. 

On the other hand, if there are fewer turns in the secondary than in the primary, then there will be a lower voltage. However, more current could then be gotten from the secondary, again depending on the ratio of turns. This is analogous to a mechanical lever, where higher speed can only be at lower force. 

Ordinarily, the primary coil of the transformer used in this course would be plugged into a 120 V ac wall socket, and only 12 volts of ac would be available in the secondary, because that has ten times fewer turns than the primary has. Thus, this transformer is usually in a step-down mode, and in fact it was used that way in the optional experiment on page 98 of the previous chapter. 

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