Electric current comes in many forms: current in a wire, flow of ions between the plates of a battery and between plates during electrolysis, as arcs, sparks, and so on. However, here on Hackaday we mostly deal with the current in a wire. But which way does that current flow in that wire? There are two possibilities depending on whether you’re thinking in terms of electron current or conventional current.
In a circuit connected to a battery, the electrons are the charge carrier and flow from the battery’s negative terminal, around the circuit and back to the positive terminal.
Conventional current takes just the opposite direction, from the positive terminal, around the circuit and back to the negative terminal. In that case there’s no charge carrier moving in that direction. Conventional current is a story we tell ourselves.
But since there is such a variety of forms that current comes in, the charge carrier sometimes does move from the positive to the negative, and sometimes movement is in both directions. When a lead acid battery is in use, positive hydrogen ions move in one direction while negative sulfate ions move in the other. So if the direction doesn’t matter then having a convention that ignores the charge carrier makes life easier.
Saying that we need a convention that’s independent of the charge carrier is all very nice, but that seems to be a side effect rather than the reason we have the convention. The convention was established long before there was a known variety of forms that current comes in — back even before the electron, or even the atom, was discovered. Why do we have the convention? As you’ll read below, it started with Benjamin Franklin.
To give you some idea of just how early we’re talking about in the field of electricity’s development, the Leyden jar, the first capacitor, had just been invented in 1745. Word of it, and other discoveries were spreading rapidly through letters and lectures. One such lecturer was Dr. Archibald Spencer. Franklin attended his lectures and even bought Dr. Spencer’s equipment in 1746.
Franklin was a prolific and rigorous experimenter and began writing his own letters about his work and his theories. It’s through those letters that we have the details of the experiment from which we get our direction for conventional current.
In a few letters he described an experiment with persons A, B anc C. Persons A and B stand on wax to insulate them from the ground, whereas C stands directly on the ground. Person A rubs a glass tube against his hand and, as Franklin describes it, “collects the electrical fire from himself into the glass”. B then passes his knuckle near the glass tube and “receives the fire which was collected by the glass from A”. But to C, both A and B appear electrified “for having only the middle quantity of electrical fire, receives a spark upon approaching B,” or “gives one to A, who has an under quantity”. If instead, A touches B then the spark is stronger because the difference between them is greater. If after A and B touch, C touches either of them there is no spark because “the electrical fire in all is reduced to the original equality”.
Franklin’s letter then continues by defining some new terminology and establishing the convention that we use today.
“Hence have arisen some new terms among us: we say, B, (and bodies like circumstanced) is electrified positively; A, negatively. Or rather, B is electrified plus; A, minus. … To electrify plus or minus, no more needs to be known than this, that the parts of the tube or sphere that are rubbed, do, in the instant of the friction, attract the electrical fire, and therefore take it from the thing rubbing: the same parts immediately, as the friction upon them ceases, are disposed to give the fire they have received, to any body that has less.”
Thus, Franklin came up with the idea that charge is something that moves from the positive to the negative, or from that which has more to that which has less. That’s the conventional current that was adopted and that we use today.
Note that by rubbing objects together as described in the letters, they’re making use of the triboelectric effect to charge the objects. Just which objects get charged positively, giving up electrons, and which get charged negatively, taking the electrons, is listed in a table called the triboelectric series. From the letters, Franklin correctly deduced which charge different objects will get, glass being charged positively and sulfur negatively, for example.
The problem is that when you get a spark from going near the positively charged glass, Franklin guessed that the electric fluid moved from the positive glass to you, whereas we now know it’s you that give electrons to the glass.
Ebenezer Kinnersley, who was a part of Franklin’s close circle of electrical experimenters is also often credited with this idea so it’s hard to know if only one person came up with it or if it was a result of a collaboration. Franklin seems to hint at the latter when in the letters he writes “And we daily in our experiments electrify bodies plus or minus, as we think proper.”
Faraday’s Current-Direction Dilemma
In the 1800s, Michael Faraday ran into similar problems of having to name charge carriers without having a full understanding. He’d done some experiments with electrolysis and, while working on a paper about them, needed names for what we now call the cathode and the anode.
The two plates of his electro-chemical cell were connected to an electrical circuit and so there was a positive plate and a negative plate. As we saw above, the convention was that in the circuit around the cell, the current left the positive plate and entered the negative plate. After deciding what to call the plates, electrodes, he then needed to distinguish between the two from the point of view of how the ions inside were interacting with them. He also wanted names that were fairly independent of theory.
He looked to an analogy with the Earth’s magnetic field and the direction the current would have to run around the Earth in order to create the fields — that would be the same direction as the sun, east to west, or going up in the east and going down in the west. His friend, William Whewell, suggested kata, Greek for downwards, and odos, Greek for a way, i.e. the way which the sun sets. The result is “cathode”. Similarly using ano, Greek for upwards, resulted in “anode”.
Interestingly, in the same paper, after offering these names, he shows his concern in naming things while it was still early days in their understanding. He writes “and whatever changes may take place in our views of the nature of electricity and electrical action … there seems no reason to expect that they will lead to confusion, or tend in any way to support false views.” Sure enough, due to the discovery of the electron and the fact that the moving charge carrier’s direction is actually the opposite, it’s been suggested that kata odos, the way down, can now be interpreted as the way down into the cell, i.e. where the electrons enter the cell.
Thomson’s Discovery Of The Electron
The discovery of the real charge carrier in a wire, the electron, started out with research into cathode rays. Cathode rays were first observed as a glow emitted from the cathode in a rarefied gas. In the 1870s, Sir William Crookes produced the first cathode rays in a high vacuum and showed that they moved from cathode to anode. He also used a magnetic field to deflect them and realized that they were negatively charged.
But it was J.J.Thomson in 1897 who realized that the rays were actually unique particles and made good estimates for the particle’s charge and mass. He called them ‘corpuscles’ but their name was later changed to ‘electron’. Thomson also found that they are what are being given off by incandescent light and by the photoelectric effect and it wasn’t long after that they were found be the charge carrier for electricity in wires.
Does It Matter?
It turns out that whether you use conventional current or electron current doesn’t matter, as long as you’re consistent in your use. Kirchhoff current law, for example, says that the sum of the current going into a junction (node) in a circuit is the same as the sum of the current going out of the junction. It doesn’t care which directions are in and out, as long as you keep track of the signs.
However, conventional current is represented in the shapes of various components in schematics. The ‘arrowhead’ shape of a diode points in the direction of conventional current, as do the ‘arrowhead’s in transistors. But it’s easy to remember that electrons flow against the arrows. The right-hand rule also uses conventional current when figuring out the direction of the Lorentz force or the direction of the magnetic field around a current carrying wire. So it seems you do at least have to be familiar with conventional current.
The Winner’s Circle
Which did you first learn? Which do you prefer? Do you use conventional current for some things and electron current for others? In my experience producing corona discharges across air gaps, it matters whether or not the sharp electrode is providing the electrons since the resulting coronas are produced differently. Share your experience and opinions in the comments below.