MOTION MOUNTAIN

20 1 electricity and fields
nylon ropes
water pipe
or tank
nylon ropes
metal cylinders
metal wires
bang!
metal cans
Challenge 4 s
Ref. 5
F I G U R E 6 A simple Kelvin generator; the one on the right lights a fluorescent light bulb using dripping
water (photograph © Harald Chmela).
giving out loud bangs. Can you guesswhat condition for the flowhasto be realized for
thistowork?AndwhatdidOpeldotorepairthecarstheyrecalled?
Ifwestopthewaterflowingjustbeforethenextsparkisdue,wefindthatbothbuckets
are able to attract sawdust and pieces of paper. The generator thus does the same that
rubbingamberdoes,justwithmorebangforthebuck(et).Bothbuckets,andtheattached
metalpieces,arethussurroundedbyelectricfields.Thefieldsincreasewithtime,untilthe
sparkjumps. Just after thespark, the buckets are (almost)without surrounding electric
field. Obviously, the flow of water somehow collects something on each bucket; today
wecall thiselectriccharge.Wealsosaythatsuchbodiesareelectricallycharged.Thisand
otherexperimentsalsoshowthatchargecanflowinmetals.Whentheelectricfieldsare
highenough,chargecanalsoflowthroughair, leadingtosparksorlightning.
Wealsofindthatthetwobucketsarealwayssurroundedbytwodifferenttypesofelectricfields:bodiesthatareattractedbyonebucketarerepelledbytheother.Thediscovery
thattherearetwo different typesofelectricchargeisdue totheuniversal geniusCharles
Dufay (b. 1698 Paris, d. 1739 Paris). In a long and careful series of experiments he confirmed
that all materials he could get hold of can be charged electrically, and that all
chargescan be classifiedinto twotypes.He wasthefirstto showthatchargedbodies of
the same charge repel each other, and that bodies of different charge attract each other.
Heshowedindetailthatallexperimentsonelectricitycanbeexplainedwiththesestatements.
Dufay called the two types of charges ‘vitreous’ and ‘resinous’. Unfortunately,
Dufay died at a young age. Nevertheless, his results spread quickly. A few years later,
Georg Bose used them to develop the first electrifying machine, which then made the
exploration of sparks and the science of electricity fashionable across Europe.*
* In fact,the fashion still goes on. Today, there are many additional ways to produces sparks or even arcs,
i.e., sustained sparks. There is a sizeable subculture of people who build such high voltage generators as a
hobbyathome;see,forexample,thewebsitewww.kronjaeger.com/hv. Thereisalsoasizeablesubcultureof
people whodothisprofessionally, paid bytaxmoney:thepeople whobuild particle accelerators.
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net