Because loads often rise and fall together across large areas, power often comes from distant sources. When the contact printer is plugged into an outlet the safelight comes on. KODAK AMATEUR PRINTER- 1914. A contact printer for negatives up to 3 3/4 x 5 1/2 inches in size . The optimum size of a conductor for a given voltage and current can be estimated by Kelvin’s law for conductor size, which states that size is optimal when the annual cost of energy wasted in resistance is equal to the annual capital charges of providing the conductor. Thus, reducing the current by a factor of two lowers the energy lost to conductor resistance by a factor of four for any given size of conductor. For a given amount of power, a higher voltage reduces the current and thus the resistive losses. Joule’s first law states that energy losses are proportional to the square of the current. The reduced current reduces heating losses. These companies developed AC systems, but the technical difference between direct and alternating current systems required a much longer technical merger.
And since conductance is proportional to cross-sectional area, resistive power loss is only reduced proportionally with increasing cross-sectional area, providing a much smaller benefit than the squared reduction provided by multiplying the voltage. Higher order phase systems require more than three wires, but deliver little or no benefit. As an example, to adjust the flow of AC power on a hypothetical line between Seattle and Boston would require adjustment of the relative phase of the two regional electrical grids. Tesla’s discovery of the rotating magnetic field produced by the interactions of two and three phase alternating currents in a motor winding was one of the most significant achievements of the century and formed the basis of his induction motor and polyphase system for the generation and distribution of electricity. The first three-phase alternating current using high voltage took place in 1891 during the international electricity exhibition in Frankfurt. Practical three-phase motors were designed by Mikhail Dolivo-Dobrovolsky and Charles Eugene Lancelot Brown. These were induction motors running on polyphase current, independently invented by Galileo Ferraris and Nikola Tesla.
Widespread use of such motors were delayed many years by development problems and the scarcity of polyphase power systems needed to power them. The development of superconductors with transition temperatures higher than the boiling point of liquid nitrogen has made the concept of superconducting power lines commercially feasible, at least for high-load applications. The 20th century’s rapid industrialization made electrical transmission lines and grids critical infrastructure. Because of the economic benefits of load sharing, wide area transmission grids may span countries and even continents. HVDC is necessary for sending energy between unsynchronized grids. Renewable energy sources, such as solar photovoltaics, wind, wave, and tidal, are, due to their intermittency, not considered to be firm. While the price of generating capacity is high, energy demand is variable, making it often cheaper to import needed power than to generate it locally. Since power lines are designed for long-term use, Kelvin’s law is used in conjunction with long-term estimates of the price of copper and aluminum as well as interest rates.
Long-distance transmission is typically done with overhead lines at voltages of 115 to 1,200 kV. Cable television signals use only a portion of the bandwidth available over coaxial lines. Congress directed that FM be the means of voice transmission for television. The first transmission of single-phase alternating current using high voltage came in Oregon in 1890 when power was delivered from a hydroelectric plant at Willamette Falls to the city of Portland 14 miles (23 km) down river. This device is a Microtel Enterphone, manufactured in the Brockville plant in February, 1985. The Enterphone was designed for apartment entry control, and unlike other systems on the market which required an intercom in each apartment and separate wiring for the intercom system, the Enterphone placed a call to the tenant’s regular telephone, saving considerable cost for the apartment owner. The cost of high voltage transmission is comparatively low, compared to all other costs constituting consumer electricity bills. It is generally made of copper as it is easily available in nature with low cost.

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