Human battery exhibit

The exhibit you describe has four large metal plates, one each copper and aluminum on each side of a meter. The copper plate on the left is connected through the meter to the aluminum plate on the right and the other two plates are connected in the reverse direction. The meter measures micro amperes, or millionths of an amp of current, so it is very sensitive.

Placing one's hands on plates connected across the meter causes the needle to deflect. This is because we, the human, are acting as the electrolyte in a battery. The two metals have different affections for their electrons, and the one that is greedier steals electrons from the one more generous through our bodies. This causes an imbalance between the two plates which is corrected by the current through the meter.

A typical visitor gets a modest reading on the meter. Sometimes a person is able to easily pin the meter at one end of its range. There might be a number of reasons for this. Larger hands and greater pressure produce more current. Moister hands work better than dry hands. I don't know if this is a real affect, but I have an impression that it is common for women to get higher readings than men. I can't imagine an explanation for this, and I am pretty good at making stuff up. I doubt it is because the average man has a greater wingspan than the average woman, but I really don't know. There is also probably a day-to-day variation with an individual depending on personal chemistry, hydration, sweaty palms, etc. I have never studied that, either.

Some of my favorite questions are those I can answer, "I don't know."

Durable Hand Generators

We build hand generators from DC gear motors which we find on surplus electroncs websites.  Look for DC gear motors and order a small selection of different ones to test. These are surplus and always change, so if you find one that you really like, buy a bunch. While you are at the electronics store you may want to pick up a pair of large alligator clips and about 18 inches of lamp cord (Two strand wire) per generator.

We get our crank handles from Reid Supply, 3.5” aluminum cranks, the part number is something like CH-35AL (pretty obvious how they coded that!) These don’t have holes in them so they fit any motor shaft size.

 We drill a blind hole (not all the way through) in the handle and fasten it to the motor shaft with epoxy. If we can, we open the motor case and solder the heavier cord directly to the connectors. If we can’t, we will solder the wires together, either way we provide some form of strain relief. we have glued the motors into PVC pipe with the cord knotted and passing through an end cap. Sometimes we have to grind off a metal flange to make a motor more hand-friendly. The alligator clips are soldered to the ends of the wire. We try to keep the wires and the colors of the clips polarized the same way so that saying, “Let’s try connecting all the red clips together” almost has a predictable result. (It depends which way the student turns the crank.)

These generators last a long time, we don’t ever strip the gears in them, and the cost of parts is less than $20. The epoxy joint is typically the failure point and very easily repaired.

We use our generators mostly to turn other generators. Students love it. We have been tempted to make some winches, with pulleys instead of cranks to see if we can lift stuff, and if we can turn the generator with falling weight. Two pulleys on the shaft; a big one and a small one? We have seen a similar device used to split water with electrolysis to make little hydrogen-oxygen explosions. Cool!

Human Battery Question

You ask if there would be an electrical current if one of the plates were made of rock.

The short answer is no. Unless the rock is a very rich metal ore, rock will not conduct electricity well enough to allow a current.

When we put our hands on the metal plates, we complete a circuit, electricity flows from one plate to the other through our arms and body, and then back to the original plate through the micro-ammeter via wires. The current, which is very small, is caused by the different way the metal plates react to the moisture in our skin; one collects electrons and the other gives them up. Any two different conductors will serve the purpose. Ours are copper and aluminum; but they need to be conductors in order to complete the electrical circuit. The human body is a pretty good conductor.

Most rocks are insulators and can't complete the circuit.

Van de Graaff generator questions

Where did you get the big globe? What is it called?

          The machine is a Van de Graaff generator, named for its inventor, Robert Van de Graaff. We bought ours (we have four of them) from a scientific supply company. They are very cool machines. Do you agree?

Why did the big rubber band break? Did electricity cause the break?

          There are two things that make the belts break. One is friction. The belts run for a long time quite fast over the rollers and this makes them wear out, and has nothing to do with the electricity.

          The other thing is a direct result of the electricity. There are tiny sparks inside the machine all the time. These sparks form a chemical from the oxygen in the air called ozone. Ozone is a strong chemical, and it attacks rubber, breaking it down and weakening it. This happens to all our rubber belts sooner or later and we have to replace them. The used, broken belts are usually pretty crumbly when we take them out of the Van de Graaff generator.

Why did you turn the machine on and off with the stick?

          This is an EXCELLENT question! The stick we used is made of plastic, which doesn't conduct electricity. The switch is on the base of the machine, which is separated from the ball by a plastic column. The plastic column is also an insulator and doesn't conduct electricity. When the generator is running, electrons are pulled from the base and added to the ball. This gives the base and the ball opposite charges, and it can be a lot of electricity.

          Working with and around the generator builds up a charge on the operator, and touching the base often gives us shocks. Using the insulated stick helps prevent some of those shocks because it doesn't conduct electricity. We aren't really touching the switch. It is a matter of avoiding getting another shock. We get enough doing this demonstration as it is.

Please tell us more about your custom Hand Generators.

Motors and Generators

These are DC gear motors I bought for about $8 apiece on the surplus market and are no longer available from my source. I am experimenting with other motors to compare their behavior with these, which seem to be working quite nicely. The gear motors came from All Electronics. The aluminum crank handles came from Reid Supply, they arrived with solid hubs and I drilled the ¼” blind holes in them. I then used two-ton epoxy to fasten the handles on the shafts. A vice or a clamp is helpful to encourage the shafts to mush all the way down into the epoxy. The motors came with four wires, two of which went to either a tachometer or a clutch of some sort. I cut the extra wires and soldered them together with the motor wires to extend the wires’ reach. Shrink tubing helps support the connections. Heavy alligator clips from Radio Shack completed the assembly. If I were to rebuild these, I’d give each motor at least two feet of wire and stagger the clips so they would be less prone to shorting themselves. Clips with plastic caps on the handles are a luxury to be considered. Avoid hooded clips, they will restrict your possibilities for connections.

At the motor station, I typically ask a student to ‘crank me out some electricity’ on a motor that is not attached to anything. Questioned, she will report that it is easy. I then short circuit the motor by touching the clips together. Suddenly the cranking is not so easy any more. I explain that now we have a circuit, and electricity (electrons) is (are) flowing through the wires. When there is no circuit, we are not moving electrons, not doing work, and indeed we all know that it is easier not to do work than it is to do work.

Next, I connect two motors together and let the student explore what this does. Often I find this is too early to try to explain that a motor and a generator are interchangeable depending on where the energy comes from and how it is being changed. A motor takes in electrical energy and changes it into motion; a generator takes mechanical motion and changes it into electricity. I also show them that if a student cranks a generator and I grab the spinning handle on a motor, the person cranking can feel me holding the motor. This is why we pay for electricity, and should give us renewed respect for batteries and power plants. At this point I step back and let them experiment.

Students will connect motors in series circuits and in parallel circuits, they will discover on their own that they can get shocks, and they will find that the motors can ‘break dance.’

I try to show them that no matter how fast they crank one motor, they can’t get another to go that fast. That would defy the Second Law of Thermodynamics; entropy always increases. (Friction being as relentless as death and taxes.) If they have a series circuit, though, they can add their voltages together to increase the speed of a third or fourth motor. This does not work in a parallel circuit with each of the motors connected to each of two common nodes. In the parallel circuit, the voltages don’t add, although I think the currents do, suggesting that the driven motor(s) might have more torque. Voltage governs speed in these motors.

Students typically don’t realize they can make the motors arm wrestle, and that is fun to show them, taking this opportunity to show them that switching wires can change the direction of a motor’s rotation. They will also discover that there are differences of effect depending on which way they crank and whether they have parallel or series circuits.

By now the time is usually used up, but if students are motivated, we explore series-parallel arrangements. Connect ‘generators’ in series to a set of parallel ‘motors’ for best effect. Interested students will discover that they only get shocks if they are touching the metal while they are disconnecting clips and someone is cranking. Show them that if a series circuit is broken anywhere else, they don’t get a shock. Show them that the shock is current and not static electricity. The shock doesn’t happen when we are just holding two clips. Connecting them together diverts any current through the wires, but disconnecting them results in a sudden mild pulse of electricity.

Inside each motor is a coil of copper wire. When current flows through this coil, it sets up a magnetic field. When the current stops as, for instance, the circuit is broken, this magnetic field collapses, sending a sudden but short-lived spike of electricity through the wire.

The hand generators are an experiment, and I see them as being in destructive testing. I am amazed at how well they have lasted, the only repairs being two handles needing reattachment to their shafts. The clip leads are holding up well, the gear boxes seem to be doing well, and after numerous drops to the floor from table height and plenty of time break dancing, the motors seem to be durable. I feel very lucky to have found these particular motors at a great price, the plastic cases are a plus as they bang around on tables.