Sir Francis Ronalds

Sir Francis Ronalds was way ahead of Faraday. He had an intuitive grasp of induction that was decades ahead of his peers.  In 1823, Ronalds wrote "...the probability that the electrical induction which would take place in a wire enclosed in glass tubes of many miles in length might amount to the retention of a charge or at least might destroy the suddenness of the discharge."  He was the first to find that electrical signals were retarded when passing through an insulated wire, or run underground.

His name was still being invoked by engineers half a century later. He was possibly the first man of science to write about induction with any level of understanding. He knew that an electrical signal passed down a wire did not do so instantly, and that it was induction that slowed it's passage. In that same era Charles Wheatstone calculated the velocity of transmitted electrical signals. He was wrong. Wheatstone had not yet learned that the diameter of the wire effected the speed of the signal.

In 1816 Francis Ronalds erected two wooden frames to accommodate eight miles of iron wire for an experimental telegraph. This unit operated entirely on what the Journal of the Society of Telegraph Engineers called in 1870 "frictional electricity." Ronalds called it a "perpetual electrophorous."  It was in fact an electrostatic telegraph whose power was generated by synchronous revolving discs. By sending a current up the wire he was able to manipulate dry balls of pith  in a Canton pith ball electrometer. He also had a hot and hooked the ends up to pistols to set them off in perfect sync.


Since this predated Morse code, Ronalds had to improvise his own method of signalling. This was not quite as clever as his telegraph. He charged the wire and then grounded it at lengths indicated by a lettered dial matching a receiving dial at the other end. The dials excluded the letters J, Q, U, W, X and Z. It was inaccurate and slow. However he tested it at distances of up to 500 feet. This is amazing for an era when static electricity, galvanic electricity, dynamic electricity, and voltaic electricity were considered to be at least somewhat separate forces.

Ronalds lived long enough to see much of what he predicted to come true. he was knighted in 1870, three years before he died. While much of his work was original, in one of his pamphlets he gave credit to a number of now obscure figures: Hans Christian Oersted, Carl August Steinheil, Lord Cavendish, Tiberius Cavallo, William Watson and many others.

DMM, VOM, DVOM, TVM & VTVM

Every radio engineer (and most any geek of the tech variety) needs two things: A soldering iron and a decent Multimeter. I've seen them called DVM for Digital Volt-Meter, or DMM for Digital Multi-Meter. Older engineers sometimes call them a VOM, an acronym for Volt Ohm Meter. Once I saw it in a catalog as a DVOM, Digital Volt-Ohm Meter.  Actually there are enough acronyms for these to just make a short list:

DVM - Digital Volt Meter
DMM - Digital Multi-Meter
VOM - Volt Ohm Meter
DVOM - Digital Volt-Ohm Meter
FET VOM -  Field Effect Transistor Volt Ohm Meter
FET VM - Field Effect Transistor Volt Meter
TVM - Transistorized Volt Meter
VTVM - Vacuum Tube Volt Meter 
DVTVM - Digital Vacuum Tube Volt Meter 

 This tool measures all the basic units of electrical power: AC voltage, DC Voltage, resistance, capacitance, ohms, and current. Nicer units will also measure frequency. Actually most geeks have a nice one and then a cheap crappy one for doing things that they know might immolate the device. Cheapo units are generally less accurate which can really matter when you're modifying a more complex circuit, like something you might connect to an Arduino. Even something as simple as a continuity test requires a multimeter. This can be used to test cables, fuses but also board soldered components. Does current pass between point A and point B?  You can't tell just by looking at it.

The VTVM is obviously an older device. The first volt meters used vacuum tubes, we didn't call them VTVMs because there were no transistorized devices. So that acronym was born later to distinguish those early analog models. Early VTVMs are about the size of a small guitar amp. There were numerous branded models. RCA had one absurdly named the "Voltohmyst." There were some early 50s units that actually were DVTVMS, employing both analog and transistor components. In the 1960s Heathkit made models you could assemble yourself. The use of that acronym dates back to at least 1949 even though the first truly digital voltmeter wasn't invented by Andrew Kay at Non-Linear Systems until 1954.  More here.

The first volt meter was a primitive galvanometer first invented by Hans Oersted in 1820. Oersted was just describing the effect of current on a compass needle. Later that year Johann Schweigger developed a moving coil galvanometer. Devices descended directly from this design were later used in the first telegraphs to detect the signals passing through the wire. It's been about 290 years since those first devices and now you can buy a cheap volt meter at a truck stop for under $10.

Twisted Pair and Schweigger's Coils

Let's rewind.

Hans Christian Ørsted determined in 1820 that the electrical charge in a wire deflected an external magnetic field. This is because when you pass an electrical current through the wire it emits it's own magnetic field. This matters because if you coil that wire the field of the adjacent windings overlap with each other. This cancels out electromagnetic noise from sources outside of the wire. Thus was born the twisted pair US patent # 220791. Thank you Alexander Graham Bell. More here. We still use twisted pair cable in Ethernet cable today. It's been over a century and the design remains in use. But Bell didn't invent twisted pair cabling until 1881, That's 60 years later. There was no direct link between Oersted and Bell. It was 1880. They never met and he couldn't just Google it.

Hans Christian Ørsted was Danish, and big on chemistry, physical and post-Kantian. He was also one of the earliest thinkers to describe the "thought experiment."  He also discovered Aluminum.  He was a clever bloke in other words. The oersted (Oe), a unit of magnetic H-field strength, is named after him.  It's a nice way to be remembered. In 1820 during a lecture, Ørsted noticed a compass needle was effected by the toggling on and off of the current form a battery. It proved a connection between electricity and magnetism. That is as far as he got.

Literally a week later André-Marie Ampère began a series of papers describing mathematically how this worked. He also later demonstrated that two parallel wires could be attracted or repulsed by one another, depending on what direction current flowed through them. More here. Shortly thereafter Johann Schweigger deduced that multiple turns of wire amplified the effect the electromagnetic effects of electrified wire on a compass needle. We now call that a Schweigger's coil. Schweigger was a professor of chemistry at the University of Halle in Germany. Ampère  used simple galvanometers like this to measure his experiments.at the time they were called multipliers. Another German, Johann Poggendorf built a multiplier in 1821. University of Cambridge professor James Cumming built a galvanometer with the addition of a magnet to offset the magnetic pull of the Earth. These designs inspired scientists like Prof. Joseph Henry and William Sturgeon to build electromagnets. More here. By 1828 Henry had made magnets that could lift hundreds of pounds. With twisted wires he made simple electric motors. One such diagram describes the wiring

"The galvanic magnet A B is wound with three strands of copper bell wire, each about twenty-five feet long; the similar ends of these are twisted together so as to form two stiff wires q r, which project beyond the extremity B, and dip into the thimbles s t."

Another pre-Bell telephone inventor (there are a few) named Antonio Meucci may have also invented twisted pair. In one of his patent drawings from 1858 he indicated two pairs of two wires each. It is unclear if they were twisted, but it's also not clear why he would have configured a 4-wire circuit for any other reason. It's equally notable that patents by Elisha Gray do not indicate multiple wires on each path. So he is another early contender.

Now you don't actually have to twist the wires together to get some of these noise-rejecting EM effects. Just winding them helped, and even just having them close together did as well. It is highly probably that some engineers running telegraph lines were aware of the noise reduction. But they didn't patent it, or didn't think of it as patentable.  So look at that image at the top of the post. The outer coil is coiled, and the inner wire a set of three twisted wires. That's an image of a marine telegraph line, the original Atlantic cable, run by Mr. Cyrus Field, completed in 1858. It was a Kerite cable. More here. It predates Bell by 20 years but it's still not a twisted pair. Close but no cigar. 

The humble toroid

I was asked months ago how a toroid removes noise from an audio cable. This is absurdly complicated so if the technical stuff eludes you, come back tomorrow. I'll avoid equations, but there is going to be some math.

In short, a toroid is a ferrite doughnut. Sometimes they are made of maganese-zinc alloy, or nickel-zinc. I can't really stop there because that wouldn't be interesting or arcane. So hold on to that doughnut idea. I'll come back to it. First we have to visit the 1800s long before Krispy Kreme.
In 1820 Hans Oersted discovered that electric currents can create magnetic fields in conductive material. Seems simple now, but this was hot news then. Andres Ampere got very excited about this. He began determining the relationship of electric currents to magnetic fields. Ampere went on to write a fundamental law of electromagnetism, Ampere's law states that the magnetic field around an electrical element is proportional to the electrical current that is it's source. This was groundbreaking and also happened to be wrong. He wasn't totally wrong, more incomplete. But he was wrong enough that there also exists an equation referred to by the catchy name of "Maxwell's correction to Ampere's law."

So in any circuit the sum of the length elements times the magnetic field in the direction of the length element is equal to the permeability times the electric current enclosed in the loop. The magnetic field depends only on the amount of current enclosed by the loop and not on how that current is distributed. In other words, the magnetic field outside the wire depends on the amount of current, not on the diameter of the wire.

Regardless the equation governs the properties of a magnetic field in a solenoid (coil of wire) a straight wire, the field inside the wire and yes even the humble toroid. Yes, we're back to the doughnut now. It is a ring of ferrite, aka iron with wire coiled around it making an inductor. This is a resonant circuit because of its self-capacitance.

This inductor acts as though it includes a parallel capacitor, because of its closely spaced windings. These adjacent windings have different electrical potential, but also lie in each others magnetic fiends. These different potential behave like the plates in a capacitor and storing charge. (Storing charge makes it like a capacitor. ) In twisted pair cable the two currents, in the toroid are equal and opposite and therefore null themselves so no radiation takes place at the cable. The two wires are thus balanced. It becomes a balun. This is how it reduces RFI.