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From: jbn@glacier.STANFORD.EDU (John B. Nagle)
Newsgroups: sci.electronics
Subject: Re: How are power line voltages determi
Message-ID: <17445@glacier.STANFORD.EDU>
Date: 12 May 88 16:22:09 GMT
References: <5770004@hpscdc.HP.COM> <162700012@uiucdcsb>
Reply-To: jbn@glacier.UUCP (John B. Nagle)
Organization: Stanford University
Lines: 38


      Part of the answer to this question goes back to the early history of
the electric industry.  In the early days of electric lamp manufacture,
Edison's lamp plants were unable to manufacture lamps of uniform resistance.
The plants of the 1880s and 1890s manufactured lamps with carbonized paper
filaments.  The carbonization process produced widely varying product.
The original target operating voltage was 100 volts DC, but the lamp 
manufacturing process resulted in lamps with proper operating voltages
between about 85 and 135 volts.  This resulted in a serious yield problem.
Only about half of the lamps would give full life at full output
at 100 volts.  This yield problem was doubling lamp production cost.
The solution chosen was one very familiar in the semiconductor industry --
part selection.  Lamps were tested with a manual photometer as the voltage
was adjusted, and the lamps thus sorted by operating voltage.

      This created a marketing problem.  What to do with the off-voltage
lamps?  The answer was to provide some diversity in system voltages.  Bear
in mind that in the early days, most power systems were isolated, and most
electric lamps were provided by the power company (a tradition carried into
the 1970s in a few areas of the U.S.).  So various systems ran at different
voltages, ranging from about 90 to 130 volts.

      Standardization came later.  Through the 1950s, there were large
installed systems with non-standard voltages, and even non-standard
frequencies; parts of Boston, for example, ran on 25Hz into the 50s.
Large parts of New York and Chicago were on DC through the 50s.  
Electronics of that vintage could usually cope; the standard radio was
"AC-DC", and many early transformerless TV sets would run happily on
DC.  In an era where 20% resistors were normally 20% and 5% was "precision".
all TV sets had large numbers of front and back panel adjustments, and 
could usually be tweaked into operating despite voltage variations.

      See "History of the Early Electrical Manufacturers", published by
the Harvard Business School, for more than you ever wanted to know 
on this subject.  MBAs may find the business plan for Edison's first
power plant fascinating.

					John Nagle