<P> In the early days of electric power usage, widespread transmission of electric power had two obstacles . First, devices requiring different voltages required specialized generators with their own separate lines . Street lights, electric motors in factories, power for streetcars and lights in homes are examples of the diversity of devices with voltages requiring separate systems . Secondly, generators had to be relatively near their loads (a mile or less for low voltage devices). It was known that long distance transmission was possible the higher the voltage was raised, so both problems could be solved if transforming voltages could be cheaply performed from a single universal power line . </P> <P> Much of early electricity was direct current, which could not easily be increased or decreased in voltage either for long - distance transmission or for sharing a common line to be used with multiple types of electric devices . Companies simply ran different lines for the different classes of loads their inventions required, for example, Charles Brush's New York arc lamp systems required up to 10 kV for many lamps in a series circuit, Edison's incandescent lights used 110 V, streetcars built by Siemens or Sprague required large motors in the 500 volt range, whereas industrial motors in factories used still other voltages . Due to this specialization of lines, and because transmission was so inefficient, it seemed at the time that the industry would develop into what is now known as a distributed generation system with large numbers of small generators located near their loads . </P> <P> High voltage was of interest to early researchers working on the problem of transmission over distance . They knew from elementary electricity principle that the same amount of power could be transferred on a cable by doubling the voltage and halving the current . Due to Joule's Law, they also knew that the capacity of a wire is proportional to the square of the current traveling on it, regardless the voltage, and so by doubling the voltage, the same cable would be capable of transmitting the same amount of power four times the distance . </P> <P> At the Paris Exposition of 1878, electric arc lighting had been installed along the Avenue de l'Opera and the Place de l'Opera, using electric Yablochkov arc lamps, powered by Zénobe Gramme alternating current dynamos . Yablochkov candles required high voltage, and it was not long before experimenters reported that the arc lamps could be powered on a 7 - mile (11 km) circuit . Within a decade scores of cities would have lighting systems using a central power plant that provided electricity to multiple customers via electrical transmission lines . These systems were in direct competition with the dominant gaslight utilities of the period . </P>

When was electricity first used for commercial purposes