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1.5.12 - STUDY & RESEARCH |
Build an aluminium-leaf Electroscope for dramatic demonstrations to large audiences.
Picture shows the assembled electroscope with a 'counter electrode' - this is the foil sphere on the green straw passing through the upright pillar.
The pointer shown here is an early design with a strip of cooking foil inserted in a large diameter drinking straw. The Foil must connect inside with the pivot pin, and protrude enough to make contact with the counter electrode. Another design is shown on the page describing the experiments.
Some of the accessories: (left to right)
- yellow straw - test negative charge
- yellow balloon and dropper for charged water drops
- green straw with drawing pin as proof plane
- rectangular foil 'cap' (alternative to blue can at top)
- card with cottons and fuse wire
- plasticene with pins
- pink balloon for source of charge
- foil sphere (ping-pong ball) on yellow straw handle
Ready made components for this instrument are available from the author.
Be assured that this is a serious instrument developed after years of teaching A-level Physics, and that with it electrostatics experiments can be convincingly demonstrated to a large audience.
CONTENTS
- Demonstrator's Tips
- List of parts required
- Assembly of the Electroscope
- Assembly of accessories
- Sources of charge
- Variations from Specifications
- Instructions for Experiments
DEMONSTRATOR'S TIPSAttention to the following points may avoid the disappointment of demonstrations not working:
SERIOUS WARNING!
- 1. A damp atmosphere can cause excessive leakage of charge. Avoid foggy or humid weather! Try again tomorrow!. Remember your breath contains plenty of moisture so keep your distance! Gas heaters may cause condensation. A hair dryer may be helpful to dry off critical insulators like the 4mm socket, but this is not usually the problem. The steamy atmosphere of cooking means that the kitchen table is probably not a good place to experiment!
- 2. The table on which the apparatus is placed must be 'earthed'. The floor surface may be vinyl or carpet which is insulating, or the table top may be formica etc. Use a 'virtual earth' (large capacity to earth) by spreading newspaper (a conductor for these purposes) over the table top. A wooden table on a wooden floor is probably well earthed.
- 3. The DEMONSTRATOR must not acquire a charge, so it is best to have one hand on the table whenever possible. Footwear often has insulating soles!
- 4. Attention should be paid to clothing. Some clothing generates static easily, which may disturb experiments. Other clothing may be useful for rubbing against BAG or BALLOON, etc., to produce a desired charge.
- 5. Ions in the atmosphere can cause leakage. Avoid naked flames, candles, burners of any kind.
- 6. Avoid draughts! Don't move around suddenly. Avoid blower heaters, fans, open windows, proximity of ventilation system outlets etc.
- 7. Damp or sweaty hands may contaminate surfaces which have to be dry to provide good sources of charge. Worse, if hands have been in contact with some anti-static preparation, smears of this may ruin the performance. Don't scratch your head (brilliantine!) or stroke your chin (after-shave!)
The high voltages produced in these experiments: about 1 kilovolt per division on the scale or 6 kilovolts full scale deflection of the pointer, would be dangerous if it were not for the minute amounts of charge involved. It must be emphasised that electricity is dangerous, and high voltages can jump gaps so don't point at them. Don't discharge things by touching them directly - use a very high resistance like a spill of paper, or long wooden rod or ruler, to avoid the small spark that occurs if a naked finger is used! If possible use a proper earth rod which is connected to true earth by a wire and hold it by a long insulated handle. That is the safe and proper way to experiment!
PARTS LISTNote: the pointer is 150mm long (6 inches) from pivot to tip.
STAND
1 Upright pillar: white plastic tube, 44mm outside dia (see picture). 3mm slots in the end for the feet, 3mm slots for the top support, and a hole about 6mm dia for the counter electrode.2 Feet for stand: 350mm x 30mm x 3mm hardboard, with interlocking slots (egg-box fashion)
1 Top support: 250mm x 25mm x 3mm hardboard strip, end slotted 9mm wide for 4mm socket
1 Quadrant of wood: to wedge feet to pillar
INSTRUMENT
1 Shaped wire frame (see picture) soldered to 4mm PTFE socket. The wires are bent to form a stirrup for the horizontal pivot pin1 Scale: green/yellow PVC sleeving (electrician's earth sleeving about 2mm dia.), 26cm long
12cm black 12mm PVC adhesive tape for marking scale
1 Pointer: with fitted pivot pin and balance pin, details in text.
1 Extra pin for zero adjustment
Reel of PVC tape to affix scale to pillar, etc.
ACCESSORIES (for instrument)
1 Pin: 'lightning conductor'1 Cap: 90mm x 30mm x 3mm hardboard with central 4mm hole
100mm x 100mm square aluminium foil (cooking foil) to wrap cap
1 M4 pan-head screw, nut and 2 washers to fit cap
1 Can: with small hole in bottom, details in text
1 M4 pan-head screw, nut and 2 washers to fit can
ACCESSORIES (for experiments)
1 Polythene straw for 'proof plane' handle1 Small Drawing pin for proof plane
Lump of plasticine: for fixing handles etc.
2 Table tennis balls
2 squares aluminium foil 100mm x 100mm to cover balls
2 Polythene straws for handles
2 Pins to help fix handles to balls
1 Polythene capillary tube, 5cm: charge source. This is obtained from the inside of heavy duty 72 ohm coaxial cable, removing the outer covering and braid, and removing the inner conductor. Probably in the scrap box of any local radio amateur - or e-mail me for a bit!
1 Balloon to serve as a water reservoir
1 Balloon as charge source
or: Any pieces of bubble-wrap packaging as charge sources
1 Polythene food-bag as charge source
1 Plastic cup for a stand
1 Polythene straw as test charge
1 Spare straw in case of accidents1 Square 100mm x 100mm of aluminium foil for capacitor
1 Square 150mm x 150mm of polythene sheet (ex food bag) for capacitor1 wire about 20 SWG to fit easily in coax cable piece, with eye, 6cm, for coaxial capacitor
1metre each of wire/cotton/polyester
1 Drawing pin for Earth connection to feet
Sum totals:
6 Pins, as used in needlework (including 3 for pointer)
4 Aluminium foil squares 100mm x 100mm cooking foil
2 Drawing pins
6 Plastic straws (including 1 in pointer)
2 M4 pan-head screws with nuts and 2 washers
ASSEMBLY of the ELECTROSCOPEThe FEET are two strips of hardboard approx 350 x 30 x 3 millimetres. They interlock by slotting one into the other about 125mm from one end, and then stand on their long edges. The white surfaces should face each other if you have that sort of hardboard!.
The PILLAR is the white plastic pipe approx 425 mm long and 38mm internal diameter (standard plumbing material as for basin wastes). The crossed slots in one end fit the crossing of the feet, so this is the bottom of the pillar. The two slots at the top of the pillar are at different distances from the top end, about 3mm only, and on opposite sides. The higher of the two slots is at the front of the stand, so face this slot between the long ends of the feet when making the slot and when fitting the pillar to the feet.
The WEDGE is a small piece of wood, quadrant shaped. Its curved edge fits the inside curve of the pillar, the two straight sides fit between the feet. Fit the wedge inside the front of the pillar by inserting it carefully - without undue force which could break the feet where they slot together, or excessive bending outwards of the plastic pipe. Some springing out of the pipe is necessary to provide the wedge action. When the wedge is in position the pillar should feel reasonably firm and stand upright and steady on its feet. A suitable diameter cork shaped with a penknife wold do as an alternative.
The TOP SUPPORT is a strip of hardboard approx 250 x 25 x 3mm with a 9mm slot in one end for the 4mm socket. The other end should be inserted from the front of the pillar into the slot at the top, and through the other slot at the back. It should project at the back about 2 or 3cm. Because the slots are at different heights, the strip will not be level until loaded with the weight of the fittings at its end.
The 4mm SOCKET must be PTFE insulation, (RS Components or Maplin etc). The socket which is soldered to the wire frame fits into the slot at the end of the top support. The hardboard is fitted between two large washers, with the wire frame downwards. Slide the socket to the end of the slot. Tighten the nut, finger tight is enough. The bent wire ends soldered together at the bottom of the wire frame should point towards the pillar.
The FRAME is made of two 18 SWG wires soldered at the top to the 4mm socket, bent to separate them to be parallel about 12mm apart, and bent to join together again at the bottom where they are soldered for about 20mm. This is then bent at right angles towards the Pillar as an anchor for the plastic sleeving scale. The SCALE is a length of green/yellow PVC sleeving about 26cm long. It should be marked with black PVC adhesive tape cut from the bobbin supplied with the kit. Use a 2cm length of tape for each marker. The first should be wrapped round so the centre of the mark is about 2cm from the end of the scale. This will be the zero mark at the bottom of the scale. Five more marks should be added spaced 3cm centre-to-centre. Fit the zero end of the scale over the bent wire ends at the bottom of the wire frame. Curve the scale upwards towards the pillar and secure its other end to the pillar temporarily by a strip of PVC tape.
The POINTER is the most delicate and important part of the instrument. It is made from a plastic straw covered with a strip of aluminium foil spiral-wrapped around it. The start of the foil is secured by a slit in the end of the straw. The end is held in place by a wrap of PVC tape, the length of which forms part of the counter-balance weight. The rest of the balance weight is a small piece of plasticine stuck to the end of the straw, in which is a pin that can slide in or out. This pin provides a fine adjustment of the balance. The pivot pin is fitted 150mm from the end of the pointer. It rests in the cradle provided by shaped bends in the wire frame. Insert the tip of the pointer throught the wire frame towards the pillar. If the pre-set position of the balance pin becomes disturbed, readjust it so the long end of the pointer hangs freely downward and returns there after deflection without being unduly bottom-heavy. Handle the pointer carefully at its PVC tape when lifting it in or out of its cradle.
ZERO ADJUSTMENT is provided by a pin which has to be inserted in the prepared holes. With the head of the pivot pin to the left, one hole is just above it by about 2mm. Take care not to loosen the pivot pin during the following work. Insert a pin sloping downward to emerge from the other side opposite the centre of the pivot pin. The zero-pin slopes downward about 45 degrees with its point towards the pillar. Push it through about half way and adjust it in or out until the pointer hangs at rest opposite the zero mark of the scale. Check that the tip of the pointer keeps about 1cm away from the scale throughout its deflection from vertical to horizontal. If necessary adjust the attachment of the top end of the scale to the pillar. Don't obscure the hole through the pillar when positioning the scale. It may be necessary to slide the top support in or out a bit to get a good relationship of pointer to scale.
ASSEMBLY OF ACCESSORIESThe CAN is a 'Coke' or similar aluminium can with its lid removed. CARE! THERE MAY BE SHARP EDGES INSIDE WHEN THE LID HAS BEEN REMOVED. It depends on how the lid is cut out! But sharp edges inside will not affect the performance. Every effort should be made to smooth the edges, but avoid putting fingers inside just in case. MORE CARE! The strength of the can relies on its lid and the pressure inside. An empty can without its lid is easily damaged.
Fit a 4mm pan-head screw through a hole in the bottom of the can from the inside. The shakeproof washer fits inside the can under the screw head. Tip: place handle of screwdriver on table with its blade pointing up. Balance the screw with its washer on the tip of the screwdriver. Add a tiny bit of plasticine if needed. Lower the can onto the screw viewing its position through the hole. The flat washer fits outside under the nut. Tighten firmly. A cross-head screwdriver is helpful. It is desirable that the can is water-tight at the screw! Check that the screw in the can fits into the 4mm socket to provide a secure support. Small dents in the can may be pressed out from the inside using a rounded object like a screwdriver handle, but their presence will not affect the performance.
The CAP is formed by wrapping the small rectangular piece of hardboard, 90 x 30 x 3mm approx, in a 100 x 100mm square of aluminium foil. Smooth the foil flat and fold it as necessary.
Insert a 4mm pan-head screw with shakeproof washer under the head through the foil and the hole in the centre of the hardboard. Add the flat washer and nut and tighten. Check the fit of the cap on to the 4mm socket.An excellent PROOF PLANE is made by attaching a drawing pin to the end of a plastic straw. Use a tiny ball of plasticine pressed into the end of the straw, and press the drawing pin into it.
Two CONDUCTING SPHERES are required.
A table-tennis ball is first wrapped in foil. Tip: place a foil square in the palm of a hand. Press the ball into the centre of the foil, cupping the hand and pressing the foil up around the ball with the fingers. Take care not to tear the foil. Complete the wrapping by smoothing the wrinkles in the foil flat against the ball surface. Attach a handle by a small piece of plasticine and a pin. Tip: hold a pin upright with its head on a firm surface. Place a ball over it and press the ball down until the pin enters about half way. Press the plasticine around the pin and its head and carefully press the straw onto it so the pin head goes inside the straw. Press any surplus plasticine against the foil around the end of the straw. If handled with care, the attachment should be quite adequate for all the experiments.The COUNTER ELECTRODE required for some experiments uses one of the conducting spheres. The plastic straw handle is passed through the hole in the pillar from the front so the ball is positioned roughly opposite the end of the pointer and a short distance from it. An earth connection has to be made to the ball by taping onto it a length of wire or cotton which is connected to the feet (earth). A drawing pin may be pressed into the top edge of one of the feet, and the connection simply tied to it. The word 'counter' is to suggest a counting operation - the number of times the pointer touches the ball in a given period e.g. 10 seconds.
SOURCES OF CHARGE
- 1. The polythene food BAG in the kit is convenient for most experiments. When rubbed/stroked with dry fingers or cotton hankie etc. it acquires a 'negative' electric charge. Fold it into a postcard size wad for easy handling.
- 2. A rubber BALLOON when blown up (at least partially) can be rubbed/stroked with the fingers to give it a 'negative' charge.
- 3. The polythene capillary WATER-DROPPER produces drops having a 'positive' charge.
- 4. A plastic STRAW is 'negatively' charged when lightly stroked with the fingers.
- 5. The ABS plastic of which the pillar is made is readily charged by handling. This will disturb the operation of the instrument. Discharge it (or any other plastic) by GENTLY wiping (once or twice) with a damp cloth or damp tissue.
- 6. Most household plastics, especially a COMB, will acquire a 'negative' charge by friction.
- 7. PERSPEX acquires a 'positive' charge, but true perspex may not be readily available.
- 8. An ideal charge source is polythene 'bubble' wrap, the type with smaller bubbles being best. A piece like a small handkerchief can be folded into a pad or rolled, and stroked with the hand, or wiped against clothing. Excellent!
VARIATIONS FROM SPECIFICATIONS
There is nothing magic about the specification of the stand. Melamine coated hardboard looks nice, the ABS pipe is easy to cut, but one can use a metal laboratory 'retort stand' with bosses and clamps to improvise a stand. But keep the 4mm socket and the whole pointer assembly well away from the metal to keep capacitance of the instrument low.Aluminium is much to be preferred to steel for the CAN. It is so much easier to work, and doesn't rust. Technically the shape of the 'COKE' can is IDEAL - well worth persevering to get a lid off neatly. Try piercing a close line of holes with a school drawing compass all round, and then using pliers in a twisting action to tear along the dotted line (similar to winding the handle that opens a sardine tin etc). A drill press can be used with a spike to force through the holes, or a very small drill used to bore them. A small electrical grub screwdriver with its blade ground into a V-point and handle removed, mounted in a drill press, is very good as the V point first pierces a hole and then widens it into a slot to join up with the previous hole. Saves the pliers job. Cut the lid off from inside the rim as the smooth rim is technically very desirable - all rough edges should be towards the inside of the CAN.
The straws must be polythene. They must have perfect insulation and readily acquire a charge when stroked. Test using a charged traditional gold-leaf electroscope: touching the (uncharged) straw must not leak away the charge at all.
Go to Instructions for Experiments
Registered Charity No. 236962
revised 8/7/02 by WS
Wilfrid Sollom OSB, Douai Abbey, Upper Woolhampton, Reading, Berks. RG7 5TQ
e-mail to douaiweb@aol.com