Franklin's Electric Spider

Electrical demonstrations were a major craze during the latter part of the Enlightenment era, with many in high society enjoying its “shocking” effects.

In addition to his scientific pursuits, Benjamin Franklin also enjoyed having fun with electricity. In a letter from 1747, he wrote,

We suspend by fine silk thread a counterfeit spider, made of a small piece of burnt cork, with legs of linnen thread, and a grain or two of lead stuck in him to give him more weight. Upon the table, over which he hangs, we stick a wire upright as high as the phial and wire, two or three inches from the spider; then we animate him by setting the electrified phial at the same distance on the other side of him; he will immediately fly to the wire of the phial, bend his legs in touching it, then spring off, and fly to the wire in the table; thence again to the wire of the phial, playing with his legs against both in a very entertaining manner, appearing perfectly alive to persons unacquainted. He will continue this motion an hour or more in dry weather.

(Earlier in this same letter, Franklin proposed referring to charges as positive and negative. So as was common for Franklin, he blended the serious with the playful.)

You can also make and enjoy something much like Franklin’s counterfeit spider.

1. Tinsel, or linen/cotton thread, or metallized thread
2. Silk, nylon, or polyester thread
3. A small ball that can be made electrically conducting
4. A source of high voltage

The legs of the spider are formed of conductive thread. Linen and cotton tend to absorb enough moisture from the air to be reasonably conductive, at the high voltages of this demonstration. Of course, tinsel (made from aluminized mylar) is highly conductive. Whatever you choose, it should be reasonably "floppy." If you find your thread is not sufficiently conducting, you may rub it against a soft pencil lead, to deposit some graphite (which conducts well enough).

The spider must be suspended by something long, thin, and insulating. Silk was among the best flexible insulators available during the enlightenment, but most synthetic fabrics perform better today, and are more widely available.

For the ball, Franklin used a piece of burnt cork. Why burnt? To give a coating of black carbon, which is a reasonably good conductor. You can get the same effect by painting anything with high-carbon ink (India ink). Many things could be used, as long as they’re fairly light – but not too light (as Franklin noted by incorporating “a grain or two of lead”).

Franklin had an assortment of electrostatic generators. For projects like this, which relate to historical apparatus, my favorite is a miniature Wimshurst generator. Those shown in the photos are the SparKIT. But you could use an electric fly swatter, or a negative ion generator, or even clothes straight out of the dryer – anything strong enough to produce a spark. (Note: I'm not affiliated with the makers of the SparKIT or the YouTube video linked for the fly swatter - but I appreciate what they've done and encourage you to check them out.)

Franklin used his generator to charge a Leyden jar, which caused the spider to dance “an hour or more in dry weather” – which is a great way to use yours. You could make a Leyden jar (capacitor) from a plastic bottle; but if your generator is able to operate as long as you want to watch the spider, it’s not necessary.

As noted above, you want the spider's legs to be able to "flop around." If you use standard thread from a spool, that requires the "legs" to be a few inches long. Thicker cotton floss (being heavier) and the more flexible tinsel usually works well as long as each leg is over an inch (~2.5 cm) long.

Rather than making eight separate legs, it's easier to use four pieces of thread or tinsel, and pass it through the body - leaving half on each side. It's easiest to start with some extra length that you can trim down, so get four pieces at least 8 inches or so (about 20 cm) long.

My favorite version did not require much work at all: I simply used a small (~8 mm) craft bell. Other successful versions used ~1 cm ball of loosely-crumpled aluminum foil, or a ~1 cm piece of a Styrofoam packing peanut painted with high-carbon ink to make it conducting.

For the bell, I used a knife to expand its slots, so I could fit the tinsel inside.

I slid the tinsel into the slots in the bell, crimped the bell shut, then used some cyanoacrylate (super) glue to keep the tinsel from sliding. On other tests, I used a needle to thread the threads/floss through the body.

Once the conducting parts are complete, suspend the spider from at least 12 inches (30 cm) of insulating thread. You could thread through the body or use glue - you have a lot of options, and it should be easy to accomplish.

To get the spider to dance, you need to have either two oppositely-charged objects at around the same height, or one charged object and one "grounded" object. In most situations, your body can serve as a good ground.

If you have the SparKIT, the two rods sticking up are guaranteed to have opposite charges, so it's easy to work with. If you use an electric fly swatter, you'll need to pass insulating wires to the two grids (carefully, as shown in the linked video) and position them in a way the spider can reach.

If your laundry "crackles" on taking it out of the dryer, you should be able to observe some cool effects. Bring the spider near the clothes. Once it's close enough, it should pick up a charge, and have its legs splay out in all directions. With the spider clearly repelling the clothes, bring it near your finger, or any large piece of metal - it should be attracted. Once it touches, it will be neutral again, and ready to pick up more charge from the clothes.

For static effects, I've had good luck with two options in my dryer. 1. Run only cottons in the dryer. Since the inside of my dryer is plastic, the cotton picks up a positive charge from its interaction with the plastic. 2. Tumble dry some wool things with some synthetic fabrics (e.g. polyester). The wool will gain a positive charge and the synthetics a negative charge. So, they'll have a strong static cling. Peeling them apart will leave you with positive wool and negative synthetic fabric - which the spider can dance between. (See a discussion of the triboelectric series.)

The principle is the same as in many other demonstrations, such as Franklin’s Bells, or a Volta’s Hailstorm: opposite sign charges attract, like charges repel. Effectively, it’s the same explanation as shown on my “Make a Volta’s Hailstorm” page, turned on its side.

Of course, if spiders aren't your thing, you could use almost the same materials and steps to make a nice Franklin's Bells demonstration.