Chapter 1:
A BRIEF HISTORY OF ELECTRICITY
Between 1776 and 1778 Italian physicist Alessandro Volta studied the chemistry of gases. Intrigued by an article by American Benjamin Franklin about “flammable air”, he managed after much investigation to discover and isolate Methane Gas. At the same time he refined Swedish inventor Johan Wilcke method for producing static electricity. He combined the two and devised a method to ignite methane by an electric spark in a closed vessel. Volta also studied what we now refer to as electrical capacitance, derived from such separate means to study both electrical potential (V) and charge current (Q, later I as defined by Ampere). He discovered that for a given object, they are proportional. It is for Volta’s Law of Capacitance that electrical potential has been named the Volt.
In 1779 he was made professor of Experimental Physics at the University of Pavia. While there he got into heated discussions with fellow Professor Luigi Galvani. During one of these discussions Galvani set forth a theory that if you connected a frog’s leg between two different metals, “animal electricity” would be produced. Volta would have nothing to do with this theory, but soon in the process of disproving this theory, realized that the frog’s leg served as a conductor of electricity (intended as an electrolyte), and as a detector of electricity. He replaced the frog’s leg with brine soaked paper, and detected the flow of electricity. In this way he went on to define the electrochemical series, and the law that electromotive force (EMF) of a “Galvanic cell”, consisting of a pair of metal electrodes separated by electrolyte, is the difference between their two electrode potentials. This came to be known as Volta’s Law of Electrochemical Series. This process led to the discovery by electrical decomposition (electrolysis) of water into oxygen and hydrogen by Englishmen Anthony Carlisle and William Nicholson and of the discovery and isolation of the chemical elements sodium, potassium, calcium, boron, barium, strontium and magnesium by Humphry Davy.
Galvani was still not entirely convinced of his colleague’s theories on his experiments, so Volta in 1800 invented the voltaic pile, the first electric battery to produce a steady electric current. Volta determined that the most effective pair of dissimilar metals to produce electricity was zinc and silver, but later settled on zinc and copper. The chemical process itself, made the zinc rod negative and the copper rod the positive electrode. Upon announcing this electrochemical invention he credited and praised the work of William Nicholson, Tiberius Cavallo and Abraham Bennet.
Gian Domenico Romagnosi, an Italian dilettante experimenter, philosopher, economist and jurist, experimented with Volta’s and Galvani’s theories, specifically with the voltaic pile and its magnetic influence on a compass. In 1802 he published two articles showing that electrical currents can spin the magnetic needle in a compass. This led to the theory of electromagnetism, as refined and codified by Orsted twenty years later in 1820.
In 1829 Francesco Zantedeschi, an Italian Physicist at the University of Venice, published his findings on the production of electric currents in closed circuits by the approach and withdrawal of a magenet. This was the foundation of electromagnetism. Incidentally he was also the first discoverer in 1838 of the connection between light and magnetism, and proceeded by thirty years Maxwell’s substantive electromagnetic theory of light.
Andre-Marie Ampere, a French physicist and mathematician gave us the mathematical model, and scientific procedure for the study of “electrodynamics”. He was the first to truly understand knowledge as a separate human endeavor based on mathematical models. Science would not exist as a concept without his contributions. The unit of measurement for voltaic and galvanic current, the ampere is thus named after him. Georg Ohm, the German physicist and mathematician also conducted research on Volta’s electrochemical cell. He found that there was a direct proportionality between the potential difference (V) applied across a conductor and the resultant electric current (I). The relationship is known as Ohm’s law.
The only problem with Ohm’s treatment of the subject matter, The Galvanic Circuit Investigated Mathematically, 1827, was that he was convinced that electricity was only possible in so far as it was “contiguous action”, namely that the communication of electricity only occurred between “contiguous particles”. This was of course 100% wrong. Nonetheless what interests us is his mathematical approach, regardless of the fact that his observations and conclusions were incorrect.
The first electric generator was invented by Englishman Michael Faraday in 1831. It was a simple copper disk that rotated between the poles of a magnet. It generated very low voltages because of its single current path through the magnetic field. He later found that more current could be produced by winding multiple turns of wire into a coil. This and subsequent designs produced a series of spikes or pulses of current separated by no current. The detrimental effect of large air gaps in the magnetic circuit were not yet fully understood.
French instrument maker Hippolyte Pixii in 1832 solved the issue of how to produce direct current from an alternate current source as described by Farraday. Farraday’s generator used a permanent magnet which rotated. This spinning magnet was positioned so that its north and south poles passed by a piece of iron. Pixii found that the spinning magnet produced a pulse of current in the wire every time a pole passed the coil. However the north and south poles of the magnets induced currents in opposite directions (AC current). To convert the alternating current to direct current, Pixii invented a commutator, a split metal cylinder on the shaft, with two springy metal contacts that pressed against it.
The remaining problem in creating non chemical electricity was overcoming the air gap issue in Faraday’s generator. Antonio Pacinotti, an Italian Physics Professor, solved the problem around 1860 by replacing the spinning two pole axial coil with a multipole toroidal one- essentially an iron ring with a continuous winding, connected to a commutator at many equally spaced points. This way, some part of the coil was continuously passing by the magnets. Essentially all electrical generation since has been based on that design, the only improvements being the substitution of rare earth permanent magnets with electromagnets by Wheatstone and Siemens in 1867.
The first successful radio transmission was made by Englishman David Edwards Hughs in 1879, proving Maxwell’s 1865 theory of electromagnetism. But the work of proving how this works and the mathematical model behind this was done by German scientist Heinrich Hertz. In 1886 he gave us a radio wave transmitter by using a high voltage inductive coil, a condenser (capacitor) and a spark gap between two spheres. It is because of his scientific methodology that the scientific unit of frequency- cycles per second – was named “Hertz” in his honour.
Guglielmo Marconi, another Italian inventor was the first to grasp the practical implications of this work and is credited for his development of Marconi’s Law (distance of transmission is a factor of the length and height of the antenna), and of the invention of radio transmission, the radio telegraph system and the radio as we understand it today.
The above is the abbreviated history of electricity. This would of course culminate with Enrico Fermi’s “potentials” (first nuclear chain reaction), and nuclear energy, giving us the three forms of energy as we understand them: chemical, magnetic, and nuclear.
- 2: LET THERE BE LIGHT!
The first light bulb was designed by English inventor Joseph Wilson Swan in 1860. In 1878 he patented the first carbon filament light bulb.
It was a pretty rudimentary bulb but he demonstrated all its theoretical principles in practice.
The Swan bulb of 1878.
In the same year, as was his usual practice (he would do the same with the telephone), Thomas Edison filed US Patents on Swan’s discovery and invention. In an attempt to improve on the design, and eliminate the carbon deposit within the vacuum, he introduced a second filament connected to ground. The experiment was a failure, but upon further experimentation, he noticed that the current passed to this electrode if a positive charge was applied separately. While not explaining the result or providing a theoretical framework for it, nor finding a use for it, he inadvertently produced what would later be called a diode.
In 1897 Joseph John Thomson, another English physicist properly described and demonstrated the existence of the electron. He demonstrated that you could pass current between two electrodes (at a defined distance) residing in a vacuum provided one of the two electrodes could be sufficiently heated. It is from Thomson that we got the term “Thermionic Emission”. He also described this current as a “negative charge”.
In 1889 Guglielmo Marconi, the inventor of radio communications, hired Englishman John Ambrose Flemming to work on radio reception and amplification. In 1904 Flemming finally had a working solution. The first vacuum tube was essentially a light bulb with an extra filament inside: the so called Diode. The principals were in hindsight fairly straight forward. When the white hot filament reached sufficient temperature inside a vacuum it would start emitting electrons. Provided the extra filament – the anode- was more positive than the emitting filament –the cathode- a direct current can be seen to pass between the two. The direction of the current can only be in one direction, from emitting surface to receiving surface. This was very useful in converting alternating current into direct current and for ensuring that current cannot flow in the wrong direction within a circuit.
In 1907 American Lee De Forrest patented a bulb with a major improvement on Flemming’s bulb. He added a third electrode. This electrode was a twisted wire placed between the positive anode and the negative cathode. This has since been known as a “grid”. In its simplest form the triode has only one grid, later vesions having multiple grids. A grid essentially controls the amount of electrons flowing from the cathode to the anode. Flow in our case can be simply referred to as “emission”. What De Forrest found was that a very small amount of negative current was sufficient to control a very large amount of current passing between cathode and anode. Thus he could take a very faint electrical signal either from a Marconi rig or from a wire telegraph and amplify it as he saw fit. The higher the negative current applied to the grid the lower the emission. The lower the grid voltage the higher the electron flow.
While De Forest was an inventor extraordinaire the job of working out how to explain the full physics behind this fell to two other Americans, H.D. Arnold at Western Electric and to Irving Langmuir at the General Electric Corporation.
The next innovation was to replace the simple “light bulb” anode filament with two separate components: a heater element right beside the electron emitting cathode. This allowed better control of the emission surface and a better control of the temperature. The basic need was driven by the fact that if you decrease the voltage of the heater you do reduce the current flowing, but the lower temperature meant that the emitting material was no longer at its optimal emitting temperature presenting greater difficulty in controlling emission. The heater remained at a constant voltage thus working at its maximum efficiency while the now independent cathode could be fed differing voltages with consequently controllable emission.
While not linear, higher voltage at the cathode resulted in higher current, and in turn higher voltage at the grid meant lower emission. Thus to illustrate a hypothetical triode design, 100V at the cathode could produce 100mA at the anode, but with negative 10V at the grid that would be 10mA, at 5V 50mA, at 2.5V 75mA and so forth. Again it’s a hypothetical tube and nothing in tubes is linear.
Thus a basic amplifier was born:
Assuming a basic electrical source to heat the element, Supply A
Assuming a basic electrical source capable of producing current, Supply B
Assuming a basic electrical source capable of producing a grid voltage, Supply C
Batteries supplied the voltages required by most radio sets well into the 1940’s. Three different voltages were required, thus three different batteries were called upon: the A, B and C Battery. The A battery was a “low tension” unit which powered the heating filament. Tube heaters were designed for single, double or triple cell lead acid batteries. That meant that the available voltages were limited to 2V, 4V and 6V. The high voltages applied to the cathode and anode was provided by the B battery, still to this day called the “high tension” supply. These were generally dry cell construction and typically came in 22.5, 45, 67.5, 90 and 135 Direct Current Volts (remember those voltages…). The C battery was there to provide the controlling voltage for the grid. Since virtually no current flows through the grid these batteries usually outlived the tube they fed.
In 1923 Dutch company Philips, invented the Tetrode and in 1926 the first Pentode.
The “battery eliminator” came along when rural communities started being served with alternating current in the nineteen thirties. A power supply using a transformer with several windings, one or more rectifiers themselves thermionic tubes, and large filter capacitors provided the required direct current voltages necessary. This of course raises a problem. If a tube circuit was designed to transform a AC signal into a DC signal should you not be able to test the tube within an AC circuit? Yes, but I will not concern myself with AC or Circuits or for that matter with AC Tube Circuits. Before you ask why let me just say that that would require heavy use of mathematics… still interested?
I am desperately trying to avoid using mathematics to describe events or correlations among them. They are necessary in circuit design but not necessary in tube testing. They are useful, but not necessary. More importantly they are inappropriate for the vast majority of people who do not think mathematically. I have even read that Michael Faraday was in fact a lousy scientist because his knowledge of mathematics was poor and his techne did not extend to trigonometry. To those that make such claims I answer that those that use mathematics, do so for the only purpose of confusing the examiner so as to cover up for their inability to comprehend, and overcome their cognitive deficiencies. Okay, mathematics has the advantage of brevity. Big deal, so does poetry.
So to summarize, and get back on track: A supply heats the filament, which allows the cathode to reach its operating temperature. That allows the cathode to release electrons thus freeing the potential of the B Supply, regulated by the voltage applied to the grid by the C supply.
That at its most basic is how a tube design amplifier works. The purpose of this manual is not to teach how to design an amplifier but rather to test a tube given very specific voltage parameters, in short, a parametric static test of tube variables.
The next chapter will deal with the issue of Test and Measurement of Thermionic Valves / Electron Tubes / Vacuum Tubes.
