E-mail: robert_morse@cathedral.org
AAPT Apparatus Competition, Entry Description
Low
Cost Category
Apparatus
Title: NairneÕs
Bi-polar Electrostatic generator
Abstract (50-75 words)
In the late
1700Õs Edward Nairne patented and sold a bi-polar frictional electrostatic
generator (Edward Nairne, Description and Use of NairneÕs Patent Electrical
Machine, 4th edition, London, 1793) for medical use. The
operator could set up the machine to give positive or negative sparks. The
generator described here is a modern version of NairneÕs machine made from
inexpensive materials. It can generate sparks up to 3 cm under dry conditions.
Equipment and costs required to construct apparatus:
|
Item |
Source |
Part
number |
Cost |
|
60 cm (24 inch) long base of fir 2Óx6Ó |
Building
supply store |
|
About
$0.87 ($3.49
for 8 foot length) |
|
Two 25 cm by 25 cm squares of corrugated plastic |
Craft
Store |
Strati-Core
or equivalent |
About
$1.50 ($9
for 3 ft x 4 ft sheet) |
|
30 cm length of 3 or 4 inch PVC drain pipe |
Building
supply store |
|
About
$0.50 ($5
for 10 ft length) |
|
15 cm length of 1/2 inch CPVC pipe |
Building
supply store |
|
About
$0.30 ($3
for 5 foot length) |
|
8 cm length of 3/4 inch CPVC pipe |
Building
supply store |
|
About
$0.20 ($3.89
for 5 foot length) |
|
1/2 inch CPVC end cap |
Building
supply store |
|
$0.39 |
|
two #8 by 1 inch round head machine screws
with washers and nuts |
Building
supply store |
|
About
$0.30 |
|
six 1 1/4 inch Phillips headdrywall screws |
Building
supply store |
|
About
$0.24 |
|
six #10 washers |
Building
supply store |
|
About
$0.24 |
|
four new unsharpened wood pencils |
Office
supply store |
|
About
$0.40 |
|
12
rubber bands, 3.5 inch by 1/8 inch |
Office
supply store |
|
About
$0.40 |
|
two empty soda cans |
From
trash |
|
Free
($0.10 in deposit states) |
|
8 cm
x 15 cm piece of rabbit fur or craft fur |
Leather
goods/Craft store or www.kbcrafts.com |
Item 1602 01 |
Craft
fur $0.42 (one
quarter of 9" x 12 " piece @ $1.68) rabbit
fur about $1 ( four pieces from one $4 pelt) |
|
roll of aluminum foil |
Grocery
store |
|
$1.59 |
|
two 3 inch diameter by 30 inch cardboard
mailing tubes |
Office
supply store |
|
$2.90
($1.45
each) |
|
glue stick |
|
|
$1.39 |
|
transparent tape |
Filched
from office supplies |
|
free |
|
duct tape |
Filched
from lab supplies |
|
free |
|
additional corrugated plastic or foam
plastic insulation |
Remains
from sheet of corrugated plastic or salvaged |
|
$1.50
or less |
Total Cost |
#13.14 |
||
Cost based on amount of material used. Many pieces can be found from scrap box.
Description:
Edward Nairne (1726-1806) was an English instrument maker and experimenter and later a Fellow of the Royal Society. In the 1770's he manufactured frictional electrical generators for medical use that could generate both positive and negative electric charge. The machine described here is an inexpensively constructed version of Nairne's machine. A soda can with a piece of rabbit fur or craft fur attached is rubbed by a PVC pipe rotor, leaving a negative charge on the surface of the PVC pipe. This charge is removed by corona at a sharp edge flap of a second soda can. This leaves the fur can positive and the flap can negatively charged. (Thus far this is a variant of a generator described by Morse, Teaching About Electrostatics, AAPT, 1992). The new feature of this design is that charge collection is enhanced by increasing the capacitance of the generator using two foil covered mailing tubes (Nairne used brass cylinders) as "prime conductors", in the style of Nairne's machine (Edward Nairne, Description and Use of NairneÕs Patent Electrical Machine, 4th edition, London, 1793).
The construction of the generator allows it to be used in several configurations, much as Nairne's machine, by attaching the prime conductors in different ways. (They are held on by rubber bands looped around the pencils) The prime conductors can be run parallel to the soda cans creating a gap between the positive and negative conductors. A smaller gap can be made by putting the prime conductors on the top surface of the machine and connecting them to cans with foil strips or paper-clip chains. Both may be ungrounded, or an aluminum foil ground plane may be connected to either the positive or the negative soda can. One prime conductor may simply be removed and a ground plane connected to the other can. One or both conductors can be arranged parallel to the base of the generator. The entire generator design can be scaled up, using a larger diameter PVC pipe for the rotor and larger mailing tubes for the prime conductors. A chain of paper clips or folded strips of foil may be used to transfer charge to Leyden jars or other devices. An electroscope may be attached to one or both prime conductors to indicate the charged state.
The machine described here can generate sparks 2 cm or more in length, and in dry conditions in a dark room gives a visible brush discharge from a sharp point attached to one of the prime conductors. It is intended to be an inexpensive bi-polar generator in an historical style rather than a really efficient charging device. A similar uni-polar machine in an older style is described by R. A. Morse in Franklin and Electrostatics: Ben Franklin as my Lab Partner, available at www.tufts.edu/as/wright_center along with recreations of Franklin's experiments.
Construction directions:
Build the rotor.
Fasten the 1/2
CPVC pipe to the rotor with duct tape as shown in the figure. Drill holes for
the bolts and bolt the handle in place, slide the 3/4 inch piece over the
handle and put the end cap on the 1/2 inch pipe.
Make the sides
Set corrugated
plastic so corrugations will run vertically. Mark center line, set end of rotor 2.5 cm from top edge
and draw circle around rotor. Cut out plastic with razor knife, craft knive or
other sharp tool , leaving a little extra space for rotor to turn smoothly.
Set soda cans so
left hand (collecting) can will be suspended a little clear of rotor and right
hand (rubbing) can will just about touch rotor. Mark vertical centerlines where
cans go and drill or punch four holes for pencils on vertical lines about 6 cm
above and below level of rotor and can centers. Pencils should fit tightly into
holes.
Cut notches for
mailing tube Òprime conductorsÓ on sides of plastic so that mailing tubes can
just contact sides of soda cans. (see figures)
Mount the sides.
Center sides on
2Óby 6Ó base, fasten each side to base with 3 drywall screws each with a #10
washer under head. Install four pencils by pushing pencil through top hole
onone side, putting four rubber bands on pencil, then push through to other
side. Push pencil just through bottom hole, loop rubber bands over pencil and
push through other side, so rubber bands loop top and bottom pencil. Arrange so
two rubber bands are near front face and two near back face.
Prepare soda cans
Use utility knife
to cut 2 cm flap about 8 cm long on side of one can, and bend it out. Suspend
can between two rubber bands so flap will be close to but not touching rotor,
as shown in figure. Second pair of rubber bands can press against rotor side of
can to push it slightly away from rotor.
Tape one end of
fur piece to side of second can with half wide strip of duct tape. Suspend can
between one pair of rubber bands and arrange second pair of bands to push can
against rotor. Slide rotor in between cans and adjust cans as needed.
Prepare Òprime
conductors.Ó
Smoothly coat two
cardboard mailing tubes with aluminum foil. Use glue stick to fasten edge of
foil to tube, roll foil around tube, use glue stick to fasten end of foil over
itself. Secure seam with long strip of transparent tape.
Mount prime
conductors.
Prime conductors
can be mounted perpendicular to plane of side piece using linked rubber bands
to hold them parallel to and just touching soda cans. Back ends of prime
conductors should be supported on pieces of corrugated plastic or foam plastic
Other
arrangements:
Two prime
conductors parallel to soda cans with one conductor connected to ground plane
of long strip of aluminum foil laid over base.
One prime
conductor parallel to base fastened to one can and ground plane of long strip
of aluminum foil taped to another can and running horizontally over top of
base.
Two prime
conductors parallel to base, one sticking out to left and one to right.
Operation:
Bipolar mode.
Clamp generator
base to table top. To operate generator, turn the crank clockwise. After about
ten turns or so, bring your knuckle near the foil surface of one or the other
prime conductors. Conductor connected to fur covered can will be positive,
other will be negative.
Operation:
Positive mode.
Fasten ground
plane to flap can, and prime conductor to fur can. Place hand on ground plane
while turning crank. Sparks will be positive from prime conductor.
Operation:
Negative mode.
Fasten ground
plane to fur can, and prime conductor to flap can. Place hand on ground plane
while turning crank. Sparks will be negative from prime conductor.
Connecting other
devices:
A chain of paper
clips can used to connect prime conductor to electrostatic demonstration
equipment, charge Leyden jars, etc.
Modifications:
The generator can
be scaled up by using longer cans and rotor, a wider base, and larger prime
conductors.
Sketch(es) (computer generated if possible):
