Variant 11

Most electric power stations supply electricity in the form of alternating currents. The current flows first in one direction, builds up to a maximum in that direction, and dies down to zero. It then immediately starts flowing in the opposite direction, builds up to a maximum in that direction, and again dies down to zero. Then it immediately starts in the first direction again. This surging back and forth can occur at a very rapid rate. The number of cycles completed by an electric current in one second is called the frequency of the current. In the United States and Canada, most currents have a frequency of 60 cycles per second.

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 voltage or the current may be changed to almost any value

Variant 12

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. 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.

Variant 13

A transformer is a device for changing the voltage of electricity. In popular use, it refers to the type of transformer that steps high voltage electricity down to the level necessary for domestic use. If you have electrical feed wires on telephone poles in your neighborhood, look around. Transformers often appear as large cylinders perched atop a pole, with three or four feed cables running into nearby houses.

A transformer changes voltage by means of mutual electromagnetic induction. The electrical current running through the primary coil/circuit is used to create a magnetic field. Changes in the current result in changes in the magnetic field, which in turn alters the voltage of the secondary coil or circuit. The result is that by making changes in the primary coil, the voltage of the current in the secondary coil can be either stepped up or stepped down. Transformers are perhaps the most energy efficient machines ever designed.

Variant 14

There are four common uses for transformers. Two are to step up voltage for high voltage power transmission, and then step it down for distribution and consumption. This is done between power plants and consumers, making possible the transmission of electricity over long distances.

Transformers are perhaps the most energy efficient machines ever designed. Most designs suffer very little power loss in changing voltages. The least efficient designs are actually those found in the common power adapters, such as the one attached to laptop computers. However, even these transformers are more than twice as energy efficient as an internal combustion engine, for example. The science behind transformers was demonstrated in principle by Michael Faraday in 1831. Transformers for commercial application came into their own between the mid-1880s and the early 1890s. The technology was a result of a combination of independent efforts by Mikhail Dolivo-Dobrovolsky, William Stanely and Nikola Tesla.