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In any physics class, the study of electrostatics always turns into fun. My students take an entire class period playing with the van de Graaff generator, gleefully shocking themselves, others, and making everyone’s hair stand on end. Robert van de Graaff built his prototype in 1929 and since then, thousands of students have entered their variants into Science Fairs around the world. The van de Graaff generates static charges by having a dielectric belt continually rotate around two pulleys. (A dielectric is any insulator that can be polarized, like polyvinyl, plastic, rubber, silk, nylon, or similar). Mechanical friction knocks off charges, which are then gathered onto the belt and transferred to a large conducting sphere at the top. The charges continue to build until their quantity is sufficient that they leap off onto a grounded post, resulting in a dynamic electric spark that can be up to several inches long.
The van de Graaff in most classrooms generates up to about 300,000 volts. That sounds enormous, and is, but voltage is a measure of how much energy is generated per charge. If you do not have many charges, you do not have much total energy. The charge flow involved is only about 10 millionths of an ampere, enough to produce an exciting shock, but not enough to cause injury. By contrast, US house current is nominally 120 V and 20 amperes and your typical lightning bolt is 100 million volts.
In addition to the van de Graaff, I show them a Jacob’s Ladder. In the Jacob’s Ladder, two long straight wires project vertically into space, almost parallel, but getting gradually farther apart as they rise. A spark starts at the bottom where the wires are in closest proximity. Once a spark occurs, that spark ionizes the air, heats it, and causes the ions to rise. This rising, ionized region makes it easier for the next spark to occur above the first. The process continues until the spark climbs to the top, like it was climbing a ladder. You get quite a bit of sound for your money as the spark pops and sizzles its way up. The Jacob’s Ladder is a mainstay of old Frankenstein Movies where the arcs keep climbing in the lab awaiting the genesis of the monster. I show students a clip from the 1931 classic where Dr. Frankenstein wails, “It’s alive, it’s alive.”
The name, Jacob’s Ladder, comes from the Biblical account in Gen 28: 10 – 17, in which Jacob, the son of Isaac and grandson of Abraham dreams about God. I read this to the students:
Jacob left Beersheba and went toward Haran. And he came to a certain place and stayed there that night, because the sun had set. Taking one of the stones of the place, he put it under his head and lay down in that place to sleep. And he dreamed, and behold, there was a ladder set up on the earth, and the top of it reached to heaven. And behold, the angels of God were ascending and descending on it! And behold, the Lord stood above it and said, “I am the Lord, the God of Abraham your father and the God of Isaac. The land on which you lie I will give to you and to your offspring. Your offspring shall be like the dust of the earth, and you shall spread abroad to the west and to the east and to the north and to the south, and in you and your offspring shall all the families of the earth be blessed. Behold, I am with you and will keep you wherever you go, and will bring you back to this land. For I will not leave you until I have done what I have promised you.” Then Jacob awoke from his sleep and said, “Surely the Lord is in this place, and I did not know it.” And he was afraid and said, “How awesome is this place! This is none other than the house of God, and this is the gate of heaven.” (ESV)
At room temperature, in average humidity, it requires a potential difference of about 1-3 million volts to arc a distance of one meter. That is about 25,000 volts per inch. For my class demonstration, I found a broken neon sign at a garage sale. The tubes were cracked but the transformer still worked. Five dollars put it in the back of my car. The transformer listed an output of 4,500 volts at 25 mA (25 thousandths of an ampere). Under the right circumstances, currents of even 100 mA can be lethal. The Jacob’s ladder demonstration is for knowledgeable teachers, not classroom horseplay.
I glued two wooden blocks together and attached the transformer with screws. For the conductors, I used two 3-foot welding rods I found in my garage. The nice thing about welding rods is that they are mostly iron, a relatively poor conductor, and are extremely stiff. So, they stand up tall and straight. I screwed them to the board, hooked up the wires and I was in business. At the base, the wires are only a quarter inch or so apart. But, as the spark climbs the “ladder” it gets to be almost two inches across before it reaches the top.
Here is the photo or you can click on the link to see it working. Jacob’s Ladder is easy to make and a great demonstration device. Just keep yourself and students a couple of meters away and enjoy the dancing arc as it climbs the ladder skyward.