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Alternating current has several characteristics that make it more attractive than direct current as a source of electric power, both for industrial installations and in the home. The most important of these characteristics is that the voltage or the current may be changed to almost any value desired by means of a simple electromagnetic device called a transformer. When an alternating current surges back and forth through a coil of wire, the magnetic field about the coil expands and collapses and then expands in a field of opposite polarity and again collapses. In a transformer, a coil of wire is placed in the magnetic field of the first coil, but not in direct electric connection with it. The movement of the magnetic field induces an alternating current in the second coil. If the second coil has more turns than the first, the voltage induced in the second coil will be larger than the voltage in the first, because the field is acting on a greater number of individual conductors. Conversely, if there are fewer turns in the second coil, the secondary, or induced, voltage will be smaller than the primary voltage.

The action of a transformer makes possible the economical transmission of electric power over long distances. If 200,000 watts of power is supplied to a power line, it may be equally well supplied by a potential of 200,000 volts and a current of 1 amp or by a potential of 2,000 volts and a current of 100 amp, because power is equal to the product of voltage and current. The power lost in the line through heating, however, is equal to the square of the current times the resistance. Thus, if the resistance of the line is 10 ohms, the loss on the 200,000-volt line will be 10 watts, whereas the loss on the 2,000-volt line will be 100,000 watts, or half the available power. Accordingly, power companies tend to favor high voltage lines for long distance transmission.

Alternating current has several characteristics that make it more attractive than direct current as a source of electric power, both for industrial installations and in the home. The most important of these characteristics is that the voltage or the current may be changed to almost any value desired by means of a simple electromagnetic device called a transformer. When an alternating current surges back and forth through a coil of wire, the magnetic field about the coil expands and collapses and then expands in a field of opposite polarity and again collapses. In a transformer, a coil of wire is placed in the magnetic field of the first coil, but not in direct electric connection with it. The movement of the magnetic field induces an alternating current in the second coil. If the second coil has more turns than the first, the voltage induced in the second coil will be larger than the voltage in the first, because the field is acting on a greater number of individual conductors. Conversely, if there are fewer turns in the second coil, the secondary, or induced, voltage will be smaller than the primary voltage.

The action of a transformer makes possible the economical transmission of electric power over long distances. If 200,000 watts of power is supplied to a power line, it may be equally well supplied by a potential of 200,000 volts and a current of 1 amp or by a potential of 2,000 volts and a current of 100 amp, because power is equal to the product of voltage and current. The power lost in the line through heating, however, is equal to the square of the current times the resistance. Thus, if the resistance of the line is 10 ohms, the loss on the 200,000-volt line will be 10 watts, whereas the loss on the 2,000-volt line will be 100,000 watts, or half the available power. Accordingly, power companies tend to favor high voltage lines for long distance transmission.