Question: Why is an antenna is used in this electronic [electroscope circuit](http://amasci.com/emotor/chargdet.html)?
I would like to do this as my mini project.

Answer: 
I called it "antenna" in my article, but really it's just one plate of a capacitor.

The second capacitor-plate is the entire JFET source/drain circuit, which is connected to the ground surface ...or the surface of whatever battery is being used.

The input of this JFET circuit is actually a capacitive divider circuit, with the parallel capacitor being the JFET's gate-source capacitance ~5pF, and the smaller series-input capacitor being the dielectric space between the gate lead and a charged object nearby. This smaller space-capacitance value is far below 1pF, so the input section is dividing by more than 5x.

In other words, if the gate wire is tiny, the capacitive divider is dividing too much! The FET becomes very insensitive to external e-field signals. And, if the gate-wire is several feet long, the divider is dividing too little. In that case the circuit is completely overloaded by ambient 60Hz fields, AM radio transmitters, distant thunderstorm VLF static, etc, and the LED just lights up uselessly. 

A 2cm "electrode" works fine (just use the transistor's gate lead itself.) Or, for less sensitivity, snip the gate lead shorter. Even try cutting it off entirely, that way you're still using the remaining (1mm?) gate lead inside the plastic TO92 transistor package.

On a low-humidity day, try adding a foot-long "antenna," such as an alligator clip-lead attached to the JFET gate. If the ambient 60Hz field in your location doesn't overload the circuit, you'll find that the LED still turns on and off even when you stand 15ft away and comb your hair. Try simply lifting one shoe slightly off the carpet!

Hint: connect a 10pF capacitor between the JFET's gate and source. (Or try a 3.3pf, or 0.5pf, etc.) You'll find that this visibly decreases the sensitivity, as well as slowing down the usual recovery-time. The added capacitor demonstrates the existence of this capacitive-divider effect, since you've increased the gate-source capacitance without changing the series capacitance of the space around the gate wire. Perhaps even connect a 20pF trimmer capacitor between gate and source, so you can smoothly adjust the sensitivity.

As @Hoets points out, this circuit is *TOO* cheap and simple. It doesn't give linear results or bi-polar output. (It was intended as one cheap pixel in a [300-cell display matrix](http://amasci.com/electrom/electrom.html), an e-fields physics exhibit.) A JFET op amp would work better. Try a unity-gain voltage follower circuit, using a floating input pin as the "antenna," with output to red/green LEDs or an actual panel meter display. Schematics on several other sites are listed in [the article links.](http://amasci.com/emotor/chargdet.html#15)
it's not really an antenna in the normal sense .... it's a metal so it's a sea of electrons, when there's an electric field around they all get attracted down one end leaving the other end with the opposite charge, since it's connected to the gate of the FET the FET turns on or off, depending on the charge

As mentioned above a 1M resistor on the gate would probably be a good idea