2. Outline
• Overview of Machine
– Sectors/Disks
– Neutralizing
Brushes
– Charge collectors
– Leyden Jars
– Electrodes
•How the Machine
Functions
•Use with X-Ray Tubes
•James Wimshurst
3. James
Wimshurst
• WasaBritish inventor, engineer,
shipwright. Born in 1832.
• Electrical influencemachinesgained
hisattention in 1878.
• Mademany of thepreviousmachines
already in existence; also modified
them to createintermediateforms.
• By 1883 created what isnow known
astheWimshurst InfluenceMachine.
• Never patented, but machines
becameassociated with hisname.
4. The Machine
• Themachineitself iscomposed of
several parts. Namely:
– Two counter-rotating disks
(including sectors)
– Neutralizing brushes
– Chargecollectors
– Leyden jars
– Dischargeelectrodes
5. The
Machine
• Two counter-rotating disks
– Allow for electrical potential/induction
– Must beascloseaspossibleto allow for electrostatic induction between
sectors
• Must bemadeout of non-conducting material
– Thusonly sectorscan conduct, hold charge
– Glass, plastic
• Generally moresectors= better performance
– Sectorsdistributeelectric field moreeffectively
Disks & Sectors
6. Neutralizing Brushes
• Two pairsof neutralizing brushes
• Don’t actually “neutralize”; simply
allow for induction to occur
between platesin contact (or
chargeflow depending on theory)
• Makephysical contact with the
sectors.
The
Machine
7. Charge Collectors
• Theseareableto absorb or drop
electronsfrom passing sectors
– Theresulting chargeiscarried
into external circuit (Leyden jars)
• Alwaysaresharp, thin points
• Never in contact with disk;
however, ascloseaspossible
The
Machine
8. Why must the charge collectors be
Sharp?
• Sharp pointshaveastronger
electric field for two reasons:
– They haveahigher surfacearea
per acertain volumecompared to
flat or rounded objects. Thus
they can createastronger EF.
– Chargesnaturally enjoy moving
to points
• So actual exchangeof e- occurs
between combsand sectors; must
havestrong EF, high EF density.
9. Why must the charge collectors be
sharp?
• All electronsin a
conductor want to be
positioned so net
force= 0
• They then accumulate
on apoint so asto
accomplish this
• Higher density of
charges= stronger EF
…resultsin…
10. Leyden Jars
• Simpleform of capacitor
• Collect built up chargefrom
collector combs, allowing for
largevoltage
• Inner foil contact induces
oppositechargeon outer, with
both capacitorshaving
reversed chargesfrom each
other.
The
Machine
11. Leyden Jars and Dielectrics
• Dielectricsaregenerally
non-conducting materials
(plastics, glass, etc.)
• However theatomspolarize
in EF.
• ThiscreatesaEF from the
atom in theopposite
direction, cancelling the
opposing EF to adegree.
• Thusgreater charge at a
lower voltage, increasing
capacitance
Polarization diagram
Effect in capacitor
• E = V/d : ↓E, keep d constant, V
decreases.
• Thus C= Q/V ; ↓V, ↑C.
12. Capacitance in my Jars
• Polycarbonatematerial
used asdielectric
• Certain permittivity value
(dielectric constant) in
relation to vacuum
permittivity constant (ε0)
• In thiscase εr=3.
• Greater permittivity
meansgreater polarization
and thusresistanceto EF.
C = εrε0 A/d
C= 3(8 .8 5x1 0-1 2
)(0.01 7 1 53) /
(4x1 0-4
)
A = πDL
A= π(.042)(.1 3)
A= 0.01 7 1 53 m2
C= 1 .1 38 53 x 1 0-9
F
o r
C= 1 ,1 39 pF
________________________________
13. Electrostatic Induction
• No chargesgained/lost
– Objectsmaintain overall
electrical neutrality
• Simply rearrangement of
electrons
• Coulomb’sLaw
• Critical to performanceof
Wimshurst machine
14. So How Does it Work?
• Video
• Start with small
chargeimbalance
• Inducesopposite
chargeon other plate
at brush; brush
inducesopposite
chargeon other side
of plate.
• Chargesbuilt up are
collected at combs
until max voltage
reached
15. What?
• Exact method
furiously debated;
problem with extra
charge. From
wheredoesit
come??
• From theair?
• Variety of methods
and theories
developed, still
rather fuzzy.
Most likely result of
tinkering and gradual
evolution of
machines
“ Existing theo ries o n the
actio n o f electro static
influence machines are
no to rio usly insufficient”
(Jo hnso n 51 ).
16. Discharge Electrodes
• EF and voltageincredibly
hugebetween electrodes
• Oncepolarization of
moleculesin air hasreached
limit, resultsin dielectric
breakdown
• Essentially ionization
• Gasionizesand then
current flows
• Light from recombination,
collisions(smack)
The
Machine
17. How Much Voltage?
• Dielectric strength of
air = ~3MV/m
• Thusif spark gap of
1cm, needs30,000Vto
createspark.
• Greater voltagesof
coursepossiblefor
better machines
Can calculateother stuff:
Uc = 0.5CV2
Uc = 0.5(1 .1 38 x1 0-9
)(30,0002
)
Q=CV
Q = (1 .1 38 x1 0-9
)(30,000)
_________________________
Q = 3.41 4 x 1 0-5
C
Uc = 0.51 21 J
18. • Wimshurst machineshad been
used earlier in the20th
century
to produceX-raysfor medical
purposes.
• Obviously thehigh voltage
allowsfor high energy
photons.
Use with X-Ray Tubes
_______________________________
qV= 0.5mv2
= hf
qV=hc/λ → λ = hc/qV
λ = (1.99x10-25
) / (1.6x10-19
)(30,000)
λ = 4.15 x 10-11
m
or 0.0415nm
19. Bibliography
• Clipper Controls Inc. “Dielectric Values.” <http://clippercontrols.com/pages/dielectric-values#P>
(June 5, 2010).
• de Queiroz, Antonio. “Operation of the Wimshurst Machine.” <http://www.coe.ufrj.br/~acmq/wh yhow.html> (May 30, 2010).
• Elert, Glenn. “Dielectrics.” <http://physics.info/dielectrics/> (June 7, 2010).
• __________. “Dielectric Strength of Air.” <http://hypertextbook.com/facts/2000/AliceHong.sht ml> (June 6, 2010).
• Holladay, April. “Why Charge Accumulates, and the Field is Strongest, on Pointed Ends of Conductors.”
<http://weatherquesting.com/sharp-points-why-charges-pack-into.htm> (June 4, 2010).
• Johnson, V.E. Modern High-Speed Influence Machines. London: 1922.
• Kuruts, Ron. “Electrostatic Induction.” <http://www.school-for-champions.com/science/static_in duction.htm> (June 7, 2010).
• Spiritus-Temporis.com. “James Wimshurst.” <http://www.spiritus-temporis.com/james-wimshur st/biography.html> (June 5, 2010).
• Springer, Matt. “Electric Charge and Sharp Points.” <http://scienceblogs.com/builtonfacts/2009/
06/yesterday_we_dumped_a_bucket.php> (June 3, 2010).
• von Slatt, Jake. “Jake's Wimshurst Machine and How to Build It!” <http://steampunkworkshop.c om/how-build-wimshurst-influence-
machine-part-1> (May 25, 2010).
