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One night, I was killing mosquitoes with an electric pest-killer bat. Some time later, I was using the same thing to write on paper.
Felt like I discovered fire!
HIGH VOLTAGE ALERT
It's a pretty safe experiment to perform, given that you properly understand what you're doing. So, if you want to try this on your own, go ahead. Just be careful about the high voltage though, since this bad boy can shock you with up to about 3000 volts. This can differ, depending on the brand or model you are using.
If you really want to understand what exactly is happening here, I suggest you to try this on your own. Point to be noted here, this setup cannot be used to write on ordinary paper.
Summary
Play around with the device to understand the risks and how it works.
Make the necessary connections. You will need an additional metal surface, such as a steel ruler, and a connecting (jumper) wire.
Get some thermal printer paper. This is a special type of paper used in shops and ATMs to print receipts.
Understanding the circuit
If you've ever used an electric pest killer bat, you'll know that there are three "nets" on the head of the racquet. As laymen, it's quite normal to think that the net in the middle is the one that shocks and kills the mosquito, while the two other nets on the outside are there just to protect the user from being shocked. Well, that's not the case.
Here you can see one red wire and two blue wires. The red wire is connected to the net in the middle, while the two blue wires are connected to each of the exterior nets. A transformer and a few capacitors can also be seen here. Basically, this circuit steps-up the battery voltage from around 4.5 volts to around 3000 volts across the nets.
Note: The transformer alone does not step up the voltage. Since the battery provides DC voltage, the capacitors have to play the central role in providing the large voltage across the nets.
The point is, all three nets of the racquet are connected to power, with the central net connected to the positive terminal, and the exterior nets connected to the negative terminal at ground (zero) potential. As a result, current can flow between the nets if a connection is made between them.
Now that we understand the construction of the device, let's see how the device actually works.
Sparks!
Anything, even air, can conduct electricity, if given enough voltage. It's because a very high voltage can cause air particles to become ionized, literally ripping off electrons from atoms that make up the air. The ionized air can then conduct electricity. As ions are charged, their flow will cause an electric current. You can actually think of air being a resistor of very high resistance, and applying a large voltage can overcome this resistance, enabling current flow.
A mosquito (or any bug, actually) has a lower resistance than air. When the pest happens to be between two nets of the electric bat, the voltage is high enough to cause a momentary electric current to flow from the middle net to the exterior net through the mosquito, killing it in the process. The sparks that occur are due to some air being ionized, and also the mosquito getting partially fried. (Or burnt maybe? Since there's no oil?)
In this video, I've shown how a spark may be formed in a small air gap. You may have noticed that the sparks were created when the screwdriver was touching only one of the nets, and was very close to touching the other.
Having the metal part of the screwdriver touch the interior net, the screwdriver is connected to the positive terminal of the circuit, and is at a very high voltage level (+3000 V). Sparks were generated when I brought the screwdriver close to the exterior net, which is connected to the negative terminal at 0 V. Due to this large potential difference (voltage) between the screwdriver (which is at +3000 V) and the outer net (which is at 0 V), the air in the small gap between the screwdriver and the net becomes ionized, conducts electricity, and makes the noise.
Making the connections
Attach a steel ruler (or any suitable metal surface) to the outer net of the racquet head. This acts as the negative terminal (cathode) of the circuit, and is our "table".
Fix a jumper wire to the inner net, and make sure the wire is sufficiently far from touching the outer net. The wire acts as the positive terminal (anode) of the circuit, and is our "pen".
With the bat powered, if the "pen" is brought close to the metal "table", sparks form.
When such high voltages can flow current through air and mosquitoes, a thin piece of paper shouldn't be a big deal, right?
Sparking through paper
Grab a piece of paper and place it on the metal "table", and try to write on it using electricity with the "pen" we just set up. As you do, there will be sparks, but they won't leave any spot on the paper. They will, however, leave some tiny holes on the paper, at the points where the sparks cross the paper. The point is, you see no ink, and there was no writing.
Now, if you use a special type of paper, the sparks will leave actual ink-spots on paper. This type of paper is used by thermal printers, and is used to print bills and receipts in shops and ATMs. Such paper is sold in rolls (try googling "thermal printer paper"), which of course I don't encourage you to buy for such a silly purpose. You can either tear off a piece from receipts you already have, or ask for a few centimeters of this paper from a local shop.
With such paper, shops and ATMs relieve from the pain of refilling printer cartridges, as printing can be done on this paper just by applying heat in the desired regions. When heat is applied on the surface of this paper, ink spots form on the heated regions. When sparks are created through this paper, the little heat generated by the sparks is sufficient to form spots on the paper. And hence, we can write using electricity on this very specific type of paper.
And that's pretty much it.
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