E-mail: robert_morse@cathedral.org
AAPT Apparatus Competition, Entry Description
Low
Cost Category
Apparatus
Title: Quick
Electrostatic Motor
Abstract (50-75 words)
Benjamin
Franklin built the first electrostatic rotary motor in 1748-49 using a set of
sewing thimbles on glass strip radials, powered by Leyden jars. His motor was
about 1.25 meters across. A small electrostatic motor can be quickly assembled
from a plastic champagne glass on a pencil point bearing, with brass fasteners
on the rim, powered by charged film can Leyden jars.
Equipment and costs required to construct apparatus:
|
Item |
Source |
Part
number |
Cost |
|
Plastic
champagne glass-top section |
Party
store or grocery store |
|
$0.05
to $0.10 depending on quantity purchased |
|
Small
piece of card stock |
Filched
from office supplies |
|
free |
|
Two
strips of transparent tape |
Filched
from office supplies |
|
free |
|
8 cm
by 8 cm piece of foam plastic insulation |
Salvage
from packing materials or construction site |
|
free |
|
New
wood pencil |
Store |
|
About
$0.10 |
|
16 to 20 small brass fasteners |
Office
supply store |
|
About
$0.50 depending on quantity purchased |
|
4 empty 35 mm film cans |
One
hour photo store |
|
Free |
|
50 c m length of aluminum foil |
Filched
from kitchen |
|
Free |
|
Glue stick |
Office
supply store |
|
About
$1.00 |
|
4 large gem paper clips |
Filched
from office supplies |
|
Free |
|
One or two foam cups |
Saved
from coffee |
|
free |
|
50 cm length of 1 inch PVC pipe |
Building
supply or plumbing supply |
|
About
$0.70 (at
$4 for 3 m length) |
|
Dry rough paper towel or piece of cloth or fur |
Paper
towel from roll in lavatory |
|
free |
Total Cost |
Under $3.00 |
||
Description:
Rotor:
Sharpen
pencil and poke through foam block to form vertical pivot. Cut narrow strip of
card stock to fit from rim to rim of plastic champagne glass, cut hole in
center slightly larger than pencil diameter to form loose lower bearing and
tape across from rim to rim of glass. Slip from 12 to 20 brass fasteners onto
rim, evenly spaced. Balance on pencil point. Use bits of tape to improve
balance so it spins freely.
Leyden jars:
Cut
strip of aluminum foil to fit around outside of can up to 1 cm below top, and
overlapping about 0.5 cm on bottom. Burnish foil smooth with back of finger
nail, cover with glue from glue stick, paste smoothly onto outside surface of
can and smooth over bottom of can. Unbend long leg of gem clip and poke through
center of top of film can. Fill can with water even with top edge of foil,
replace cover so leg of clip dips into water. Make four. If two are made from
black plastic cans and two from clear cans it is easier to keep track of
charges.
Assembly:
Place square of foil on table top, set rotor
assembly in middle, arrange oppositely charged film can alternately around edge
so clip comes close to but does not touch brass fasteners.
Operation:
Charge
two film cans "by the hook." Set can on upturned foam cup, briskly
rub PVC pipe with paper towel or cloth, pick up can by coating and draw hook
along outside of PVC pipe. Repeat about five or ten times. Set charged film can
on foil.
Charge
two film cans "by the coating." Set can on upturned foam cup, briskly
rub PVC pipe with paper towel or cloth, pick up can by hook and draw coating
along outside of PVC pipe. Handle can only by hook in this procedure, setting
it always on the foam cup. Repeat about five or ten times. Set charged film can
on foam cup, then pick up by the coating and set on foil.
When
all four cans are charged, two by hook and two by coating so as to have
opposite charges on the paper clips, arrange alternately around rim of rotor.
Adjust slowly until motion of rotor starts. This may take some fiddling, and
you may need to nudge the rotor to start it.
Troubleshooting:
Test
film cans for charge by small piece of foil suspended by thread. Bring foil
near hook and make sure it repels after touching hook. I t should then be
attracted to coating or to hook of oppositely charged can.
Make
sure bearing is smooth and rotor is free to turn.
Make
sure pencil is vertical.
Works
best in drier conditions.
Sketch(es) (computer generated if possible):