In the previous article we saw the history of electricity and the major milestones in its journey. In this article, we will take a look at what DC and AC means / is. We will also what the “war of currents” was all about, who fought it, when, and why?
In the figure above, current and/or voltage are plotted on the Y-axis while time is plotted on the X-axis.
- The grey line is variable current. That’s all that can be said about it.
- The blue line is pulsating current. It is varying but with a fixed pattern.
- The red line is direct current, or DC in short. It is so called as it is constant with respect to time. Batteries, that we so commonly use in clocks, watches, radios, TVs, set-top-boxes, are DC sources.
- The green line or curve is alternating current, or AC in short. It is so called because its values are changing or alternating, in a very regular way. The current starts with a value of zero, increases, reaches its peak value, and then starts falling all the way back to zero. After that, it does the same thing in the opposite direction, i.e. the values are negative instead of positive. The positive and the negative cycle together form one complete cycle, and this keeps happening over and over again. In India, we have 230V/50Hz AC supply. What that means is that the root mean square – called RMS in short, it is the square root of the average of the square of the instantaneous values – of the curve is 230V; the peak value is √2 times the RMS value, or around 325V. There are 50 cycles per second and that is what the 50 Hz refers to.
In 1879, Thomas Edison invented the incandescent light bulb and realized the need for an electrical distribution system to provide power for lighting these bulbs. He proposed a 110V DC system and switched on the world’s first electric power distribution system to 59 customers in lower Manhattan, around his Pearl Street station. But the DC system required the use of large distribution wires, which were very costly, and forced the generating plants to be close to the loads/consumers. But in many ways, that was his vision: to have many “micro” grids with their own generating plant, DC distribution system, and consumers. (Edison’s vision of many micro-grids is becoming relevant again, although in a completely different way and for very different reasons, a topic that I will touch upon much later in the series.)
Around the same time, Lucien Gaulard of France and John Gibbs of England developed a power transformer, a device that transforms the voltage level at its input to a different level at its output, and the output can be higher or lower depending upon the user’s requirements. Although transformers were around for a while, the Gaulard/Gibbs transformer could handle high power and could be manufactured readily. This transformer attracted the attention of George Westinghouse , the inventor of railroad braking using compressed air and already a legend in his own right. Westinghouse had become interested in electrical power distribution because of his interests in gas distribution and telephone switching. He was of the opinion that Edison’s low-voltage DC distribution meant high currents and serious power losses. His main objection however was that Edison’s scheme was too inefficient to be scaled up to large sizes.
So Westinghouse stuck with his idea of AC grids and built his first network in 1887, five years after Edison built his first DC system. Within a year, he built thirty more systems but the scheme was limited due to the lack of an effective metering system and an AC motor. In the year that followed, Westinghouse developed an induction meter that used a rotating magnetic field to measure the alternating current. And Nikola Tesla demonstrated a poly-phase brushless induction motor also based on a rotating magnetic field. Because of all the above developments, Westinghouse won the war of currents and the grid, as we know it, came into being.
Edison didn’t take it well at all. In fact, he opposed it bitterly, and did everything in his power to discourage the use of AC distribution systems, and AC in general. One of the things that he did was to carry out a publicity campaign to prove that AC was unsafe, including spreading misinformation on fatal AC accidents. He directed his technicians to publicly kill animals, mostly stray cats and dogs but also unwanted cattle and horses. He also tried to popularize the term for being electrocuted as “being Westinghoused”!
Edison opposed capital punishment, but his desire to disparage AC led to the invention of the “electric chair”. Harold Brown, who was secretly paid by Edison, built the first electric chair for the state of New York. It was first used on 6th August, 1890 to kill a convict named William Kemmler. However, the voltage level wasn’t high enough on the first jolt of electricity, and therefore instead of killing William it only badly injured him; the procedure had to be repeated with the “right” voltage level to achieve the “desired” effect. A reporter who witnessed the incident described it as “an awful spectacle, far worse than hanging”. George Westinghouse commented, “They would have done much better with an axe!”
The war of currents and the way Edison acted when he began to see that he was losing it, highlights how irrationally people – who are otherwise very rational and intelligent – can behave when their egos get in the way. Edison was obviously not being objective about the whole thing because beyond a certain level DC is just as fatal as AC!
In the next couple of articles, we will see what the grid, as we know it, looks like, what makes it work, and what makes it fail.
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