MOTION MOUNTAIN

liquid electricity, invisible fields and maximum speed 21
pendulum
with metal
ball
on the roof
in the hall
in the ground
F I G U R E 7 Franklin’s personal lightning rod – a copy of
Gordon’s electric chime – is one of the many experiments that
shows strikingly that charge can flow.
Challenge 5 s
Ref. 6
Challenge 6 s
Twenty years after Dufay, in the 1750s, the politician and part-timephysicistBenjamin
Franklin (b. 1706 Boston, d. 1790 Philadelphia) proposed to call the electricity created
on a glass rod rubbed with a dry clothpositive instead of vitreous, and that on a piece of
ambernegative instead of resinous.Thus, instead of two types of electricity, he proposed
that there is really only one type, and that bodies can either have too much or too little
of it. With the new terms, bodies with charges of the same sign repel each other, bodies
with opposite charges attract each other; charges of opposite sign flowing together cancel
each other out. Large absolute values of charge imply large charge effects. It took over a
hundred years before these concepts were accepted unanimously.
In summary,electriceffectsareduetotheflowofcharges. Now, all flows take time. How
fast is electricity? A simple way to measure the speed of electricity is to produce a small
spark at one end of a long metal wire, and to observe how long it takes until the spark
appears at the other end of the wire. In practice, the two sparks are almost simultaneous;
the speed one measures is much higher than everything else we observe in our environment.Howcanwemeasurethetimenevertheless?
And why did different researchers get
very different speed values in this experiment? The result of these experiments is that the
speed of electricity is typically a large percentage of the speed of light.
Sparks, electric arcs and lightning are similar. Of course, one has to check whether natural
lightning is actually electrical in origin. In 1752, experimentsperformedin France,
followingasuggestionbyBenjamin Franklin, publishedinLondonin1751, showedthat
one can indeed draw electricity from a thunderstorm via a long rod.* Thunderstorm
clouds are surrounded by electric fields.These French experiments made Franklin famous
worldwide; they were also the start of the use of lightning rod all over the world.
Later, Franklin had a lightning rod built through his own house, but of a somewhatunusual
type, as shownin Figure 7. This device, invented by Andrew Gordon, is called an
electric chime. Can you guess what it did in his hall during bad weather, all parts being
made of metal, and why? (Do not repeat this experiment; any device attached to a
lightning rod can kill.)
* Thedetailsofhowlightningisgeneratedandhowitpropagatesarestillatopicofresearch.Anintroduction
isgivenon page 204.
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net