Electricity and magnet playwiths


Capacitance effects
Electrostatic hoops
Kelvin water dropper
Leyden jar
a kitchen compass
A galvanometer
A dry compass
Other pages on this site

How to do it

The Kelvin water dropper
You will need a metal pipe ending in two fine nozzles, a supply of water controlled by a tap (that is a faucet in the USA), four metal cans, two metal funnels, some heavy wire, and a very well-insulated support system for the cans.

The set-up is that shown here. Two jets of water flow from an earthed metal pipe, breaking up into droplets as the jets fall through the two cans, A and B, then into the metal funnels in to bottom of C and D, and so into a drain.

If the insulation that you use is good enough, a charge will build up that is sufficient to make a spark flash between the upper cups, or a smaller spark gap made with copper wires, and almost joining cans A and B.

This will help you understand

You will need some aluminium foil, something cylindrical to make it into a thin flat hoop, and a charged rod of some sort. It does not matter whether the rod is positive or negative, though you may prefer not to accept that claim without checking it.

This one is simple: make a rod like an aluminium ring, and bring a charged rod near it. If you have access to a serious source of continuous static charge like a Van de Graaff generator or a Wimshurst machine, you may even be able to attract an egg ring on a dry day.

This will help you understand

Magnetic hoops?
Try hanging the same hoop by two long pieces of thread, each thread attaching to the side of the hoop. Try moving a bar magnet towards and away from the hoop, and notice what happens.

This will help you understand

Making a Leyden jar
You will need a foam ("disposable") cup, some aluminium foil, and an insulating surface. Wrap the inside and the outside of the foam cup with aluminium foil, but keep the two covers from touching each other. Leave a small "tag" of aluminium foil on the outside to allow "earthing" for charging by induction. This involves charging a rod, placing it inside the cup (not touching the aluminium!) and then touching the outside foil by way of the tag. Take your hand away, and then remove the rod: the cup is now charged.

This will help you understand the jar's operation

Making an electrophorus
You will need an aluminium cake tin about 25 cm square, a plastic handle and "Araldite" or a similar epoxy glue, a piece of rubber inner tube, and a piece of flannel or fur. Glue the handle to the inside of the cake tin with epoxy glue, and smooth off any rough edges, as these will discharge rapidly. The cake tin is your electrophorus. Put the rubber sheet on a wooden table, and charge it by rubbing it vigorously with the fur or flannel. Then, holding it by the insulated handle, put the electrophorus down on the rubber, and touch it with a finger. When you lift the electrophorus, it will be charged.

You can use a smooth metal disc in place of the cake tin, and any flat chargeable insulator will do in place of rubber: a layer of wax does very well, and gives greater historical accuracy. You can transfer a useful charge to your Leyden jar with this gadget.

This will help you understand

Kitchen compass
This compass requires a wine-cork, a sharp knife, a board, two steel pins, a magnet, a glass and some water.

Magnetise the pins by stroking them with the magnet, always moving the same end of the magnet the same way along each pin. Cut a disc of cork, fill the glass with water, and sit the disc of cork on the water.

If the cork slides to one side of the glass, add more water gently, until the meniscus bows up in the middle: now the cork will always stay in the centre. Sit the two pins on the cork, with both facing the same way, and your compass is ready to go. Though it isn't very portable . . .

This will help you understand

Measuring electricity
This is a simple gadget to detect electricity when it flows.
You have a matchbox tray, a long piece of fine enamelled copper wire, and a compass. Put the compass in the matchbox tray, coil the wire gently around the tray, always going in the same direction, and put it on a table, with the compass needle parallel to the coils of wire. When a current goes through the wire, it makes a small magnetic field, which may be able to make the compass needle twitch.

This will help you understand Back to the index

Making a small compass
You will need a press stud, three steel pins and a piece of cork, as well as a bar magnet to magnetise two of the pins. Two pairs of pliers may be useful before you are finished. Check the pins with a magnet before you start, as sewing pins are often made of brass, not steel.
Take the bottom half of the press stud, which has four holes around it, usually used for sewing it onto clothing. using the pliers, and bending the stud as little as possible, push a pin through two of the holes. Then push a second pin through the other two holes. Then stroke the pines, twenty or thirty times with the same end of the bar magnet, always starting at the pointy end and going to the pinhead end (or the other way, if you don't like following instructions).

Now the pins will be magnetised. Push the third pin through the cork slice, and balance the press stud on the tip of the pin. Move the pins so the whole arrangement is balanced, and it should turn, so that one end faces north, and the other end faces south.

This will help you understand

And now for some help

The main point to notice is that the top nozzle system is a metal conductor which is earthed. Statistically, the drops that fall from the nozzles will sometimes be charged. Once the first charged drop has fallen, everything else follows from that. The other key point to note is that the water jets must break up into droplets as they pass through cans A and B.

Suppose the first drop through can B is positive: that means that can B takes up a negative charge as electrons are attracted from can D. This charge repels electrons in the water, so that the droplets falling through B are more likely to carry a positive charge, passing this to the metal funnel below, in can C, and giving a positive charge to can C.

Some of this positive charge can now flow to can A, increasing the charge separation in the water flowing through the metal nozzle, so that more negative water passes through A, transferring its charge to can D, and so to can B, and so on.

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Why the hoop is attracted
The charged rod either attracts electrons to the near side if the rod is positive, or repels the electrons in the aluminium to the back if it is negative. In either case, this leaves the side nearest the rod with an opposite charge. And there's your answer.

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Magnetic hoops?
The principle is quite simple: moving the magnet induces a current in the aluminium hoop, and that current produces a magnetic field that reacts with the magnet's field.

The main thing to watch out for is that you are not just seeing the effects of air currents. The hoop is very light, and this can happen. Try taping the magnet to a dowel that passes right through the hoop.

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How the Leyden jar works
The Leyden jar was the first capacitor. Apparently invented in Holland at the University of Leyden, it could be used to store electrical charge until the charge was needed for an experiment.

The classic Leyden jar of the physics text is a glass jar with layers of tin foil inside and out. Ideally, the outer layer is "earthed", so it will help if you sit your jar on a metal surface. The classic Leyden jar also has a piece of metal with a ball on the end, sticking up out of the centre: this makes it easier to charge the jar and to discharge it. In this case, you will be charging the jar by contact with an electrophorus , but it is probably better to keep the whole system insulated, and to charge it by induction.

And how do you charge it by induction? Bring a charged rod or electrophorus close to (but not in contact with) some part of the inner surface, then earth the outer surface, let go, and remove the charge source.

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How to use the electrophorus
When the electrophorus is placed on the rubber, repulsion effects cause the top of the electrophorus to have the same charge as the rubber, while the lower surface has the opposite charge. If the rubber has a negative charge, electrons are pushed up towards the top, if it is positive, a few electrons are attracted down, leaving a positive charge at the top.

When you touch the aluminium, electrons flow to or from the aluminium, so the top now has no charge. When your hand is taken away, the charge from the lower half spreads out all over the aluminium, and the electrophorus has an induced charge which is opposite to the charge on the rubber mat.

The principle of the electrophorus is that charge can move through a conductor like aluminium, but it cannot move through an insulator like the rubber mat.

An old "78" record works well here, while vinyl LP records can also be used

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Kitchen compass
As you stroke the pins, always in the same direction, some of the magnetic domains in the pin are flipped over, making the whole of the pin a weak magnet.
When you add more water to the glass, surface tension pulls all of the water molecules together, forming a sort of "skin", and the cork will always float up to the highest point, thus letting as much dense water as possible flow down to the lowest point, making a more stable arrangement. Surface tension is a serious matter for living things, as you can see in Surface tension bits and pieces

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Why does the compass needle twitch?
Well, you should already know this one. The current in the coil makes a small magnetic field that pushes the needle away from where it has lined up with the earth's magnetic field, which is actually quite weak. The more turns of wire you have, and the thicker the wire, the more effect you will see, but if the wire is too thick, and there are too many turns, you may not be able to see the compass at all!!

No compass? Look at the next mini-experiment!

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How the compass works
The press stud dome only makes a very small contact with the pin tip, giving almost a frictionless support. With gravitational forces balanced, the remaining force is the force of the earth's magnetic field, and this pulls the compass around.

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This file is http://www.ozemail.com.au/~macinnis/scifun/elecmag.htm, first created on August 28, 1997. Last recorded revision (well I get lazy and forget sometimes!) was on August 4, 2001.
Worried about copyright? You need to go look at my fine print . Well, maybe you don't after you read the next paragraph, but do it anyhow . . .and to see some more ideas, look at the start of that same page
©The author of this work is Peter Macinnis -- macinnis@ozemail.com.au , who asserts his sole right to the product as it is packaged here, recognising that many of the ideas are common. Any non-profit educational or home use is completely acceptable without let or hindrance. Copies of this whole file or site may be made and stored or printed for personal or educational use. The work used here derives from on-going research and development which will one day lead to a book on brain food ideas.
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