Feel the current
To Do and Notice
Turn on the ball notice that brush shaped discharges and lightning like streamers of light connect the inside sphere and the outside surface of the ball.
At low currents the discharge spreads wide, at higher currents it collects into streamers.
A flowing electric current makes magnetic fields. The magnetic fields in turn exert forces on the current pushing it together. This "magnetic focussing" is used in particle accelerators.
Touch the ball with your hand, notice that your hand influences the pattern of the discharge.
Can you feel the current? Different people have different sensitivities for detecting current. However if you cannot feel the current it is probably less than 10 microamps.
Place a coin on top of the ball, or a piece of aluminum foil. Bring your hand near the coin. Notice the short blue sparks between your finger and the coin. Can you feel the current?
If you cannot feel the current with your fingertip, there are two places on your body that are most sensitive to electric current, your eyeball and the tip of your tongue. I don't suggest using your eyeball.
Workers on high voltage electric lines who get a large current flow through their bodies can suffer cell death and gangrene infection along the path of the discharge. Recently researchers discovered that the electric current blew holes in cell walls all along the path of the discharge. This perforation process is called electroporation. If too many cells in one portion of the body become perforated the body cannot repair them all and so the cells leak into the intercellular fluid and eventually die.
The small currents experienced in this experiment have not been shown to have any ill effects. However during the last century there was an epidemic of hysterical paralysis. People suddenly found that they could not use their hands. The paralysis was cured by applying alternating electric currents to the hands of the patients. Currents of 10 milliamps or more will cause involuntary muscular contraction. Once patients saw their hands move, then they found that they could move them again.
Hold one leg of the neon tube in your fingers. Stick the other leg of the tube out toward the ball. Notice that the light will glow even when it is not touching the ball. To glow, neon tubes need more than 8o volts across them, but they only require 10-12 amps, a picoamp. For small currents like a picoamp, air is a conductor. So electric current flows through the air and causes the neon tube to light.
Notice that the glow is between the legs of the neon tube. This indicates an AC signal. Wiggle the tube back and forth notice that you cannot see it blink on and off. The signal produced by the ball is high frequency alternating current.
Optional, if you hold one end of the fluorescent tube and bring the other next to the ball the fluorescent tube will also glow. It too requires high voltage and low current to produce light.
What's Going On?
The inner ball is coated with conducting metal as is the inside of the transparent outer glass ball. The metal on the inside of the outer glass ball is thin enough to be transparent yet thick enough to conduct electricity. A high voltage AC transformer alternately piles up electrons on the inner ball and removes them. When the pile of electrons becomes big enough, its voltage relative to the outside wall becomes large enough that electrons are knocked off the metal coating and pushed toward the outer ball. At low currents these electrons repel each other and make a glowing brush. As the current increases however the electrons are pushed together by the magnetic field created by their own flow and self organize into glowing lines of current.
When you place your hand next to the glass wall, electric charge is attracted to your hand, recall that charges attract and are attracted to neutral objects. The lines of current move toward your hand.
Electric charges in the air move in response to the alternating high voltage on the ball. these electric currents can light the neon tube.
Return to Day 15
Scientific Explorations with Paul Doherty
30 May 2000