In a con­test be­tween Edi­son and Tesla, Tesla won the bat­tle. But Edi­son may yet win the war. To prove his point he ar­ranged the pub­lic elec­tro­cu­tion of stray dogs, cats and horses.”

Cosmos - - View Point -

THE DE­CI­SIVE BAT­TLE took place in 1893 at the Chicago World’s Fair. On one side, the cel­e­brated in­ven­tor Thomas Edi­son. On the other, his for­mer em­ployee Nikola Tesla.

And what were they fight­ing over – love, re­li­gion, ter­ri­tory? None of the above. They were fight­ing over al­ter­nat­ing cur­rent (AC) ver­sus di­rect cur­rent (DC).

A quick ex­plainer: cur­rent in metal wires is the flow of elec­trons, pushed along by a volt­age. If the volt­age is sourced from a bat­tery, the elec­trons flow in one di­rec­tion only. We call this di­rect cur­rent, or DC.

How­ever, batteries are not a pri­mary source of energy. For that, we of­ten use coal or nat­u­ral gas. Their chem­i­cal energy is re­leased in a fur­nace as heat to cre­ate steam that turns the shaft of a gen­er­a­tor. In the sim­plest case, the shaft spins a mag­net in­side a coil and through the prin­ci­ple of elec­tro­mag­netic in­duc­tion pro­duces an elec­tric cur­rent. The po­lar­ity switches from pos­i­tive to neg­a­tive and back many times per sec­ond as the gen­er­a­tor shaft ro­tates, thus the cur­rent al­ter­nates in di­rec­tion. We call this al­ter­nat­ing cur­rent, or AC. Even though the di­rec­tion of the cur­rent al­ter­nates, its ef­fects do not can­cel out. The cur­rent does use­ful things in both di­rec­tions, such as heat­ing the wires in a toaster.

Start­ing in the late 1880s, Edi­son de­vel­oped a cost-ef­fec­tive means of gen­er­at­ing DC elec­tric­ity, and a suite of re­lated de­vices, in­clud­ing mo­tors and me­ters to mea­sure DC energy con­sumed. How­ever, there was a prob­lem. There was no way back then to con­vert the DC volt­age to higher or lower val­ues. To be safe for use in homes and fac­to­ries, the DC gen­er­a­tors were de­signed to pro­duce elec­tric­ity at low volt­ages. The down­side was that this meant the losses dur­ing trans­mis­sion from the gen­er­a­tor to the con­sumer were high. Edi­son judged that to be an ac­cept­able com­pro­mise, but it lim­ited the dis­tance be­tween the gen­er­a­tor and con­sumers to less than a kilo­me­tre or two.

In the other camp, Tesla had a se­cret weapon known as the trans­former. It is a sim­ple ar­range­ment of iron cores and cop­per wind­ings that al­lows volt­age to be con­verted up or down. The lim­i­ta­tion is that trans­form­ers only work with AC elec­tric­ity.

With trans­form­ers, Tesla could boost the gen­er­a­tor out­put to thou­sands of volts for low-loss trans­mis­sion over long dis­tances then cut the volt­age down again to safe val­ues for fi­nal de­liv­ery to the con­sumer.

There was a lot at stake, in­clud­ing patent roy­al­ties and the right to elec­trify the cities of the United States. The rag­ing bat­tle was called the War of the Cur­rents.

Feel­ing the tide of bat­tle swing­ing against him, Edi­son changed tac­tics and launched a mis­in­for­ma­tion cam­paign to ar­gue that AC cur­rent was dan­ger­ous. To prove his point he ar­ranged the pub­lic elec­tro­cu­tion of stray dogs, cats and horses.

These skir­mishes con­tin­ued dur­ing the lead up to the Chicago event, till vic­tory was de­clared for the Tesla AC camp. They were awarded the con­tract to elec­trify the Fair. From there it was all AC, with the de­fin­i­tive stake in the ground be­ing the 1896 elec­tri­fi­ca­tion of street lights in the city of Buf­falo with AC power sup­plied from hy­dro­elec­tric gen­er­a­tors at Ni­a­gara Falls.

AC dis­tri­bu­tion of elec­tric­ity has reigned supreme for more than 100 years. But a quiet in­sur­rec­tion is tak­ing place in our midst. Our com­put­ers, ma­chines, LEDS and elec­tric cars all run on DC. And at the ex­tremes of high power – dis­tribut­ing elec­tric­ity thou­sands of kilo­me­tres from one re­gion to the other – en­gi­neers have dis­cov­ered that the losses from a mil­lion-volt trans­mis­sion line are lower if it car­ries DC cur­rent rather than AC cur­rent.

Once again, the trans­former is the se­cret weapon, but this time oper­at­ing on DC. These new trans­form­ers take the form of elec­tronic cir­cuits that con­vert DC cur­rents up and down the spec­trum from a few volts to a mil­lion or more. Lighter and smaller than tra­di­tional ones, DC trans­form­ers make it eas­ier to in­te­grate wind and so­lar elec­tric­ity into the grid, and they re­duce the like­li­hood of fail­ures cas­cad­ing from one elec­tric­ity gen­er­a­tion re­gion to an­other.

In the com­ing decades, we may see the DC in­sur­rec­tion take hold. Not through war­fare this time – I pre­dict no pub­lic elec­tro­cu­tions of stray cats.

In­stead, it will be a sub­tle, grad­ual process. But by the turn of the next cen­tury Edi­son may well have the fi­nal vic­tory.

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