Electricity - Magnetism - Teacher's Notes  
  Measuring magnetic phenomenon is difficult. First of all, it is difficult to measure magnetic field intensity without special instruments. Secondly, it is important to maintain the measurement conditions that are required for the experiment you are performing.
If you make two coils with different numbers of turns, then use a battery to test them, you will be confused by the results. If you could measure the field intensity, you would find that the field created by the larger coil is not twice as strong. Here's why:
The more turns of wire the higher the resistance in the coil. As you'll learn in other sections (resistance), the battery forces less current through a coil that has higher resistance. The current through the larger coil will be proportionally less than the current through the smaller coil and therefore the magnetic fields may be different, but not proportional to the ratio of their turns counts.
In this experiment the battery is the problem. Batteries are a source of electrical pressure or Voltage (see the " Voltages and Currents" section. The amount of current flowing is the important parameter, not the voltage, but the current is determined by the resistance of the coil.

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