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   © by Tony van Roon

Coil/Inductance History:
First a bit of history about this remarkable invention of 'electro-magnetism'. Joseph Henry (1797-1878), was one of the many brilliant US scientists who invented and used the electro-magnetic 'coil' in his university laboratory. His low-power electro-magnet could control a make-and-break switch in a high-power circuit. Henry believed the potential was in the electro-magnet's use as a control system or device in manufacture, but he was only really interested in the science of the electricity. The electro-magnet or 'relay' was a laboratory trick to entertain his students. Samuel Morse later adapted Henry's electro-magnetic-relay device after re-designing it (and claiming it as his own by patent) using thinner wire, to carry morse-code signals over long kilometers of wire.

In 1846, Joseph Henry was professor of natural philosophy (physics) at the Henry's Electro Magnet in 1831 College of New Jersey (now known as Princeton University). He had published scientific articles on a wide variety of subjects, including electro magnetism, optics, acoustics, astrophysics, molecular forces, and terrestrial magnetism, but his reputation was built primarily on his work in basic and applied electro magnetism. Among his discoveries in electro magnetism were mutual induction, self-induction, the electro magnetic relay--enabling him to devise the first electro magnetic telegraph that could be used over long distances--and the concept of the electric transformer. He also invented the first electric motor. Henry was often referred to as the scientific successor to Benjamin Franklin. Today, it is the general opinion that Joseph Henry was the inventor of the telegraph and not Samuel Morse, who did not have a technical background to begin with. Samuel Morse adapted the ideas and inventions of Henry (and Vail) into his own and patented it, making him the owner.

It is certain that Joseph Henry was important to the history of the telegraph in two ways. First, he was responsible for major discoveries in electro magnetism, most significantly the means of constructing electromagnets that were powerful enough to transform electrical energy into useful mechanical work at a distance. Much of Morse's telegraph did indeed rest upon Henry's discovery of the principles underlying the operation of such electromagnets.

Then in 1885, William Stanley (1858-1916), Jr. built the first practical alternating current device based on Lucien Gaulard and Josiah Willard Gibbs' idea. This device was called an induction coil and was very primitive. It was the precursor of the modern transformer. His coil became the prototype for all future transformers and made practical the transmitting of electricity for consumer uses such as lighting.

William StanleyBorn in Brooklyn, New York, William Stanley attended private schools before enrolling at Yale University. He began to study law at age 21 but less than a semester later left school to look for a job in the emerging field of electricity. "Have had enough of this," wrote the 21-year-old Yale freshman in 1879. "Am going to New York." With these words, William Stanley abandoned the career pattern that his father had laid out for him - college, law school, and membership in the family law firm - and set out instead on the more risky and exciting path of electrical invention.
The decision marked the beginning of a productive career whose highlights included the invention of the modern type of transformer, and the creation of the business enterprise that was to become General Electric's Pittsfield Works.

Stanley's first job was as an electrician with one of the early manufacturers of telegraph keys and fire alarms. He then worked in a metal-plating establishment before joining Hiram Maxim, inventor of the machine gun and already a pioneer in the electrical industry. As Maxim's assistant, Stanley directed one of the country's first electrical installations, in a store on New York's Fifth Avenue. Stanley gave early evidence of his ability and enthusiasm. As his first employer, inventor Hiram S. Maxim described him: "Mr. Stanley was very young. He was also very tall and thin, but what he lacked in bulk, he made up for in activity. He was boiling over with enthusiasm. Nothing went fast enough for him." This dynamism helped him gain an outstanding reputation in the early electrical industry.

In the 1880s every system for distributing electricity used direct current (DC). But DC transmission over long distances was impractical. Transmitting at low voltage required thick wires. Transmitting at high voltage was dangerous and could not be reduced for consumer uses such as lighting. It was known that alternating current (AC) voltage could be varied by use of induction coils, but no practical coil system had been invented.

very 1st transformerInventor and industrialist George Westinghouse learned of Stanley's accomplishments and hired him as his chief engineer at his Pittsburgh factory. It was during this time that Stanley began work on the transformer. Actually the first practical AC transformer was developed by Frenchman Lucien Gaulard and Englishman John Gibbs; improvements were made at the Ganz company in Budapest. Westinghouse instructed Stanley and his assistants, Schmid and Shallenberger, to make tests to determine the commercial value of the Gaulard and Gibbs system. He also arranged to have a number of the transformers and a Siemens alternating-current generator forwarded from England to Pittsburgh. Stanley, working under the direction of Westinghouse, devised a further improvement, which consisted in securing the enclosure of the coils by making the core of E-shaped plates, the central projections of each successive plate being alternately inserted through prewound coils from opposite sides, thus permitting separate winding and consequently the better insulation of the coils. This form was further improved by Albert Schmid, who extended the ends of the arms of the E to meet the central projection. When inserting these plates the extensions were temporarily bent upward, and upon being released each plate formed a closed magnetic circuit about the sides of the coils.

In 1885, ill health almost cut short his career--some say he worked himself too hard. But it proved a disguised blessing, because it necessitated a move to his family home, Great Barrington, Massachusetts. In those peaceful surroundings, he was able to develop some ideas he had suggested two years earlier to his employer, George Westinghouse (who helped finance Stanley's lab) for a new type of transformer. This work resulted, on March 20, 1886, in the demonstration of a prototype system of high voltage transmission employing Stanley's parallel connected transformer. This system was used by him to provide lighting for offices and stores on the town's Main Street.
Stanley received a patent on his transformer: "Induction-Coil", Patent No. 349,611. These various inventions and discoveries led up within a year to commercial production of transformers of high efficiency and excellent regulating qualities. The development was a fine engineering performance in speed and in quality. The most important single contribution was by Stanley. He brought out the parallel connection in which the transformers are connected in parallel, across the constant-potential alternating-current system, instead of being arranged in series, as in the Gaulard and Gibbs connection. He obtained patents on the method, involving the construction of transformers in which the counter electromotive force generated in the primary coil of the transformer was practically equal to the electromotive force of the supply circuit. This is obvious now, but in 1886, when the principles and characteristics of the alternating current were practically unknown, it was a wonderful invention, and revolutionary in character.
On this invention Stanley's fame largely rests. Of course Stanley did not discover or invent a theory of counter electromotive force before any one else had thought of it. Such fundamental things seldom happen in invention. His claim to great and original merit rests on the discovery of a theory which was new to him and the use of it in making a structure of immense importance in the affairs of men. Briefly, all transformers now made are built upon practically the same principles as those that were developed in these early products of the Westinghouse Company.

Stanley InductorsAssisted by William Stanley, George Westinghouse worked to refine the transformer design and build a practical AC power network. In 1886, Westinghouse and Stanley installed the first multiple-voltage AC power system in Great Barrington, Massachusetts. The network was driven by a hydro power generator that produced 500 volts AC. The voltage was stepped up to 3,000 volts for transmission, and then stepped back down to 100 volts to power electric lights.
In 1890 Stanley established the Stanley Electric Manufacturing Company in Pittsfield, Massachusetts, to make transformers and auxiliary electrical equipment as well as electrical appliances. To organize it, he joined forces with two talented associates: John J. Kelley, an outstanding designer of motors: and a former Stanley laboratory worker, Cummings C. Chesney. The company was purchased by General Electric in 1903. Stanley also developed the alternating-current watt-hour meter, making it possible to measure electricity use with a high level of accuracy. Stanley with E. P. Thomson had also invented an incandescent lamp with a filament of carbonized silk. During his lifetime he was granted 129 patents covering a wide range of electric devices. William Stanley died on May 14, 1916.

Inductance (L) is the property of an electrical circuit whereby changes in current flowing in the circuit produces changes in the magnetic field such that a counter EMF is set up in that circuit or in neighboring ones. If the counter EMF is set up in the original circuit, it is called self-inductance and if it is set up in neighboring circuit it is called mutual inductance.
The unit of inductance is the Henry (H) and is defined as that value of inductance in which an induced EMF of one volt is produced when the inducing current is varied at the rate of one ampere per second. The henry is commonly sub-divided into several smaller units, the milliHenry (10-3 Henry) abbreviated mH, the microHenry (10-6 Henry) abbreviated uH, and the nanoHenry (10-12 Henry) abbreviated nH.
The storage of energy in a magnetic filed is expressed in joules and is equal to LI2/2 and the dimensions are in watt-seconds.

some inductors Mutual Inductance:
When one coil is near another, a varying current in one will produce a varying magnetic field which cuts the turns of the other coil, inducing a current in it. This induced current is also varying, and will therefore induce another current in the first coil. This reaction between two couple circuits is called mutual inductance, and can be calculated and expressed in henrys.
The symbol for mutual inductance is M. Two circuits thus joined are said to be inductively coupled.
Tee magnitude of the mutual inductance depends on the shape and size of the two circuits, their positions and distances apart, and the permeability of the medium. The extent to which two inductors coupled is expressed by a relation known as coefficient of coupling (k). This is the ratio of the mutual inductance actually present at the maximum possible value.
Thus, when k is 1, the coils have the maximum degree of mutual induction.
The mutual inductance of two coils can be formulated in terms of the individual inductances and the coefficient of coupling with the formula: Mutual Inductance Formula

Nikola Tesla Niki's fireworks Inductors in Series:
Inductors in series are, just like resistors, additive. Provided that no mutual inductance exists.
In this case, the total inductance L is: L = L1 + L2 + ... etc.

Where mutual inductance does exist: L = L1 + L2 + 2M
Where M is the mutual inductance.

This latter expression assumes that the coils are connected in such a way that all flux linkages are in the same direction, i.e. additive. If this is not the case and the mutual linkages subtract from the self-linkages, the following formula should be used: L = L1 + L2 - 2M
Where M is the mutual inductance.

"Why is this picture from Nikola Tesla here" you may ask. Well, like Joseph Henry, Tesla was decades ahead of time. Tesla had his own variety of "transformer" look-alike while experimenting with his high-voltage inventions.

Core Material:
Ordinary magnetic cores cannot be used for radio frequencies because the "eddy current and hysteresis losses" in the core material become enormous as the frequency is increased. The principal use for conventional magnetic cores is in the audio frequency range below approximately 15,000 hertz, whereas at very low frequencies (50 or 60Hz) their use is mandatory if an appreciable value of inductance is desired.

Copper Wire Table:
Below is part of an example of a so called "Copper Wire Table" with the most popular gauge #'s and associated diameter in milli-meters. I did not specify the 'Mills' or 'Capacity' specs. A 'Mil' is 1/1000 (one thousandth) of an inch.
But this information can be found in the ARRL Handbook for Radio Amateurs which is available at your local library.

Gauge     Ohms per    Diameter
(ga)     1000 feet     in mm
 18        6.510       1.024
 19        8.210        .9116
 20       10.35         .8118
 21       13.05         .7230
 22       16.46         .6438
 23       20.76         .5733
 24       26.17         .5106
 25       33.00         .4547
 26       41.62         .4049
 27       52.48         .3606
 28       66.17         .3211
 29       83.44         .2859
 30      105.2          .2546
 31      132.7          .2268
 32      167.3          .2019
 33      211.0          .1798
 34      266.0          .1601
 35      335.0          .1426
 36      423.0          .1270
 37      533.4          .1131
 38      672.6          .1007
 39      848.1          .0897

Inductor (Coil) Color Code:
Very seldom a small coil or rf-choke will have the value stamped on them. Even the larger coils don't show anything but a code. They get a 'color-code' painted on them much like resistors have but usually only 3 bands or 3 dots, unless they are made by 'Mil Spec' (Military Specification). I opted for a couple of the most commonly used types as an example. The body itself may have a variety of colors depending on the dipped substance used.
This type of coil can be found everywhere these days and I'll take them from whatever board I can. Old vcr and television boards have lots of them. Keep a couple around, just in case.

There is more information here about toroids, cores, and chokes: [Toroids and RF]

Suggested Reading:
"Handbook of Electronics Tables and Formulas", by Howard W. Sams & Co., ISBN: 0-672-22469-0
"Radio Handbook", by William I. Orr, W6SAI, Howard W. Sams & Co.,(#22424), ISBN: 0-672-22424-0
"American Radio Relay League", or 'ARRL' book. Any handbook of the ARRL of any year will do.

Copyright and Credits:
This document and tutorial is Copyright © by Tony van Roon (VA3AVR)- 2003, ALL RIGHTS RESERVED

Re-posting or taking graphics in any way or form of this project is expressly prohibited by international copyright ©laws.

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Copyright © 2003 - Tony van Roon, VA3AVR
Last updated: December 18, 2009