Marilyn's Electricity is Running Fast

Ask Marilyn ® by Marilyn vos Savant is a column in Parade Magazine, published by PARADE, 711 Third Avenue, New York, NY 10017, USA. According to Parade, Marilyn vos Savant is listed in the "Guinness Book of World Records Hall of Fame" for "Highest IQ."

In her Parade Magazine column of August 6, 2000, Marilyn claimed that electricity moves at the speed of light.

Sorry, Marilyn!

Timothy Reed <treed@ball.com> spotted the error on Friday, August 4, in the E-Marilyn Challenge, before the printed edition of Parade Magazine was even available:

Signals do not propagate along electrical wires at the speed of light. Telecommunication workers familiar with TDRs (Time Domain Reflectometers) will happily tell you that an important property of a cable is its velocity of propagation (Vp), expressed as a fraction of the speed of light. The Vp of a cable is dependent upon conductor and shielding materials and geometry. Typical Vps for standard telephone wires are in the 0.6 to 0.7 range, although some high speed coaxial data transmission cables can reach Vps of 0.99. A wire with a Vp of 0.6 would change the answer from 0.0131 seconds to 0.0219 seconds, 67% greater.

So to quote the speed of light to 6 significant digits and the distance from New York to Los Angeles to 4 significant digits, while missing a possible 67% error in the time required for the signal to arrive is to be penny wise and pound foolish. A more correct answer is that it takes between one and two hundredths of a second, depending on the type of cable used.

She also mentions that "electricity moves at the speed of light". Since the word electricity is used to mean so many different things (electric charge, electric current, electric energy) it would be more precise to talk about the velocity of the signal, or of the change in electric field. In a DC circuit, the actual electrical charge--the electrons themselves--may move no faster than a few centimeters per hour. It may take an individual electron 10,000 years to get from coast to coast. And in an AC circuit, the electrons move back and forth--the electron will never get there.

http://www.wiskit.com/marilyn/electricity last updated August 7, 2000 by herbw@wiskit.com