Activities |
| Capacitance effects |
| Electrostatic hoops |
| Electrophorus |
| Electrostatics |
| 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.
Hoops!
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. 
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. 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. 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 . . . 
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. |
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.