Induction Coils

Thomas B. Greenslade, Jr.

Kenyon College, Gambier, Ohio 43022


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One can argue that much of what we used to call “Modern Physics” stems from the intersection of two earlier technologies: high vacuum and high voltage. In this article I will discuss the induction coil, invented in the 1830s and reinvented in the second half of the 19th century, that we still use today in producing the high voltages used to excite gaseous and high-vacuum discharge tubes.



The induction coil in Fig. 1 was sold by the Central Scientific Company in 1927 for $85. Sparks 15 cm long were produced between the two needles on the top, and the output voltage was stated to be 20,000 V per cm of gap between the ends of the needles. It was made by Thordarson, known since 1895 for the manufacture of transformers. This is reasonable, as the coil is, indeed, a form of transformer that is fed with direct current, rather than the alternating current that was used in sign transformers to excite neon advertising signs.

Induction coil from Bucknell University

Fig. 1  Induction coil from Bucknell University, now in the Greenslade Collection.


The circuit diagram for a typical induction coil is shown in Fig. 2.   The transformer consists of a primary coil, P, wound on the core, D, with a relatively few turns of heavy copper wire (the coil in Fig. 1 can have a primary current as high as 8 A) surrounded by a secondary coil, S, that contains many turns of fine wire.  The core is made up of many parallel pieces of soft iron wire; if the core were solid, eddy currents in it would reduce the output voltage and heat up the system.  The rest of the primary circuit consists of a battery (typically 6 V), a capacitor, C, and a make and break contact, B, attached to a flexible steel strip.  At the end of the strip is a small slug of soft iron. The capacitor is connected across the points of the make and break contact.

Fig. 2 Circuit diagram for an induction coil, from Ref. 1.

Fig. 2  Circuit diagram for an induction coil, from Ref. 1.

Let us start by seeing how the DC input signal to the primary coil is changed into an intermittent one that can induce the high output voltage across the secondary coil.  As the circuit in Fig. 2 is drawn, the contact is closed and the circuit through the primary coil is complete. The current through the coil sets up a magnetic field that exerts an attractive force on the iron slug, breaking the circuit, reducing the magnetic field to zero and releasing the iron slug. The flexible steel strip snaps back, completing the circuit once more and the cycle repeats. The frequency of the alternating signal to the primary is mainly controlled by the mechanical properties of the strip.

Induction coils are heavy because of the amount of copper wire in the coils. Some of the weight is also due to the flat capacitor in the thick base of the instrument; usually the capacitor is made of tinfoil sheets with varnished or waxed paper as the dielectric. The French physicist Armand Fizeau suggested in 1853 that a capacitor be placed across the point to reduce sparking. In addition the contact points are often faced with platinum to reduce the erosion of the metal during sparking. It was soon discovered that there was an optimum value of the capacitance that produced a maximum output voltage. Although Henry Noad wrote in 1879 that the “functions of the capacitor are not clearly understood” (ref. 2), it became apparent by the end of the 19th century that the capacitor produced an asymmetry between the current setting up the magnetic field in the primary coil and the current as the field collapsed. The analysis of the circuit turns out to be rather complicated (ref. 3), though it is clear that the large swings in the output voltage are associated with the opening of the contacts.

And now we come to a key aspect of the induction coil: the primary coil is fed by a signal that is varying with time, but is essentially unidirectional. The time-varying EMF produced by the secondary coil is also unidirectional, and this means that it is possible to establish one electrode of a discharge tube as the cathode terminal. This electrode produces the cathode rays that became so important in the last few years of the 19th century and beyond.

Page’s and Callan’s Induction Coils

The history of the induction coil is mainly a matter of incremental improvements, culminating in the first commercially-viable instrument produced by the French apparatus maker Heinrich Ruhmkorff (1803-1877) (ref. 4).  However, the very earliest form of induction coil is often ignored.  The instrument is based on Michael Faraday’s 1831 discovery of electromagnetic induction that soon spurred a great deal of experimentation.  It now appears that in the middle of 1836 two investigators, one in Ireland and one in the United States, developed early forms of the induction coil.

The American electrical inventor, Charles Grafton Page (1812-1868), developed the induction coil shown in Fig. 3.  This example is at Allegheny College, and it appears in the 1842 edition of Daniel Davis’s Manual of Magnetism under the name of “Page’s Compound Magnet and Electrotome” at a price of about $10.00. “Compound Magnet” is the name given to a pair of concentric coils (i.e., a transformer) and the “Electrotome” is the make and break mechanism. In this case, the pivoting mechanism employs the iron slug used in the induction coil in Fig. 1 and the switch consists of an iron wire dipping into a pool of mercury.

Page’s induction coil

Fig. 3  Page’s induction coil in the Allegheny College Collection.


Figure 4 shows an example of the “Separable Helices [i.e., a transformer] and Electrotome” that is also shown in the 1842 Davis catalogue that has two make and break devices. On the right- hand side is a mechanism akin to a modern buzzer that interrupts the DC signal. There is also a steel “rasp” that is placed across the front of the base. This attached to one terminal of the battery, and a metal bar, attached to one end of the coil, is pulled across the serrations, thus making and breaking the circuit.

“Separable Helices and Electrotome”

Fig. 4  “Separable Helices and Electrotome”, also shown on pg. 181 of Ref. 5.

Like Page, the Rev. Nicholas Callan (1799-1864) worked in a scientific backwater, in this case Ireland. In 1826 he was appointed Professor of Natural Philosophy at St. Patrick’s College in Maynooth, about 20 Km west of Dublin, and spent a fruitful scientific career in the field of electricity. St. Patrick’s College is now part of the National University of Ireland in Maynooth, and Callan’s apparatus forms the center of a splendid museum of 19th century physics artifacts on the campus.

Callen’s 1836 electromagnet with secondary coil

Fig. 5  Callen’s electromagnet from 1836 with a secondary coil wound near the poles,                    in the collection of St. Patrick’s College in Maynooth, Ireland.

In his preliminary experiments Callan took an iron bar, and wound two coils on it.  When a battery was connected to one of these coils, a momentary shock could be felt by a person holding the ends of the other coil.  Callan soon used the giant electromagnet that he built in 1836 for further experiments.  The magnet (Fig. 5) has an iron core 1.7 m long that was made by a local blacksmith.  The primary coil is wound with 240 ft of 1/6 in. copper wire.   When used as an electromagnet, this was able to lift up several tons of magnetic material.  It was converted into an induction coil by winding 10,000 ft of copper wire 1/40 in. diameter over the primary windings near the poles of the magnet.  The direct current supplied by a battery was interrupted at a rate of 2000 to 3000 times per minute using the “repeater” mechanism in Fig. 6, made by Callan from the works of an old tall-case clock.












Callan’s “repeater”

Fig. 6  Callan’s “repeater”, used to interrupt the DC signal to the primary of the induction coil in Fig. 5.

The Callan and Page coils both lack the condenser, and, as a consequence, have outputs with equal swings in the plus and minus directions. This is fine for producing a discharge in a sealed tube filled with a gas, but will not cause one of the electrodes to be positive and the other negative, as is needed for a tube that produces cathode rays.  The coils were sometimes called “shocking coils” – Callan often formed seminarians into a series circuit and shocked the entire set of them, and the future Archbishop of Dublin was said to have felt the effect of the shock for several days.

The shocking coil in Fig. 7, from a design by Charles Grafton Page, has recently been discussed in this journal (ref. 6).

Charles Page's shocking coil

Fig. 7   A shocking coil designed by Charles Grafton Page (Greenslade Collection).


Ruhmkorff-Type Induction Coils

The earliest true induction coil in the Greenslade Collection is the one shown in Fig. 8.  This bears the patent date of April 1868, and was made by Edward S. Ritchie (1814-1895), the Boston maker of “School Apparatus for courses in Natural Philosophy.  This is discussed in his 1881 catalogue in which he notes that he had devised “the mode of winding the secondary helix in strata, in planes perpendicular to the axis, requiring but slight insulation, and rendering the increase in the tension [voltage], and the length of the spark to two or three feet, practicable.”  In the nineteenth century the lack of good insulation meant that winding the coil in layers from end to end of the cylindrical form would result in sparking-over between layers.  By using many narrow bobbins of wire he was able to keep the potential difference between adjacent layers small.  Note that this device has a cylindrical glass core into which the iron core wires were placed.

Upright Ritchie induction coil

Fig. 8  Upright Ritchie induction coil in the Greenslade Collection.


Ritchie did not use the “buzzer” form of interrupter that we can see in the instrument in Fig. 1, but instead used the hand-cranked mechanical interrupter shown in Fig. 9.

Close-up of the interrupter mechanism of a Ritchie coil

Fig. 9  Close-up of the interrupter mechanism of a Ritchie coil, in the Collection of the Williston Northampton School in Easthampton, Massachusetts.


The Apps (of London) induction coil in Fig. 10 dates from about 1869, and represents the finished form of the instrument that persisted into the twentieth century.  A very similar one in the Dartmouth Collection has been discussed extensively by Pantalony, Kremer and Manasek (ref. 7).  They point out that the rotary switch on the right-hand side with the bone insulation was used to reverse the direction of the current through the primary coil; this would prevent the soft iron core from building up residual magnetism that would degrade the operation of the coil.  All of the larger coils shown in this article have similar reversing switches.  As usual, the base contains the capacitor that is connected across the contact points.  Apps rated these coils as being able to produce a spark 4 inches in length.  The Dartmouth coil cost about $80 in 1870-71; the coil in the Greenslade Collection, which came from Colgate University, has a serial number slighter lower, and is thus of the same era.

Induction coil by Apps of London

Fig. 10  Induction coil by Apps of London, in the Greenslade Collection.


The Willyoung coil (Fig. 11) on display at the Garland Collection at Vanderbilt University is truly impressive.  This coil has been discussed by the late Prof. Robert Lagemann of Vanderbilt (ref. 8) who noted that the long inner coil is 87 cm in length, and the outer (primary) coil is 40 cm long.  This device put out about 100,000 volts with an input of 10 to 20 V.  Lagemann states that this huge induction coil was bought in 1898, and this is shortly before Elmer Willyoung sold his Philadelphia business to Morris E. Leeds (which became Leeds & Northrup in 1903), and then moved to New York City to start a new business.  You can see the external interrupter on the right-rear of the base; this uses a small coil in series with the primary and a vibrating bar whose frequency can be changed by adjusting the weight up and down.

Large Willyoung induction coil

Fig. 11  Large Willyoung induction coil in the Garland Collection at Vanderbilt University.


Smaller coils dating from the early years of the 20th century are quite common.  Figure 12 shows a quartet of them from the Greenslade Collection.  The one in the lower left-hand corner, made by Gaertner of Chicago, has a primary coil that slides in and out of the secondary.  This is used to vary the effective ratio of the number of turns in the secondary to that of the primary; the resulting diminution in the spark length when there is little overlap is a measure of the reduced output voltage.  In the early years of the 20th century this was a standard device and sold in the $5.00 to $7.00 range.  The Cenco coil in the lower right-hand corner was available in three sizes: 6 mm spark for $8.00; 13 mm spark for $12.00; 25 mm spark for $21.00.  This is certainly the smallest size.  The coil in the upper right-hand corner is unmarked, but its greater thickness suggests that it has more wire in the secondary to produce a large voltage and spark.  The relatively large one in the upper left-hand corner is also unmarked, although it does have “France” stamped on its bottom.

Four early 20th century induction coils

Fig. 12  Four early 20th century induction coils in the Greenslade Collection.


Figure 13 shows a Kinraide Coil, a specialized induction coil developed in the first years of the 20th century to use as the voltage source for x ray tubes.  It is actually two induction coils, connected alternately in series so that the output voltage is doubled.  The Bakelite domes on either side are insulators, and the rotating make and break contact in the middle-front that controls the alternation is driven by the spherical DC motor by a (missing) belt.

1901 Kinraide induction coil

Fig. 13  The Kinraide induction coil from 1901.  This is in the Greenslade Collection.


Induction coils continue to be used today to produce high voltages for lecture demonstrations.  Three examples are shown in Fig. 14.  On the left is a “Ford” coil that was part of the ignition system used in the Ford Model T automobile.  This one dates from the ‘teens’ of the last century.  The coil in the middle is essentially the same thing, and was part of the original Physical Science Study Committee set of apparatus.  Macallister-Bicknell of Cambridge, Massachusetts made the coil and the other early PSSC apparatus; this coil dates from 1960. Note the 6.8 megohm resistor connected in series with the positive high voltage lead to limit the current.

L to R: PSSC induction coil, Ford coil and box-type coli

Fig. 14  From the left, a PSSC induction coil, a Ford coil and a box-type induction coil, all in the Greenslade Collection.

The coil on the right, enclosed in the sealed oak box, was made by Welch, cost $12.00 in 1928 and its “fat”, one inch spark was considered to be suitable for general laboratory and wireless work.



In 1998 Joseph Bellina of St. Mary’s College in Indiana suggested that I visit the Museum at St. Patrick’s College in Ireland. I did so that summer and paid a second visit a year later after the museum had been rebuilt. His prompting led me to a continuing interest in Fr. Callan and his work on the induction coil. A query about Ruhmkorff coils from a long-time friend, Antoine duBourg of St. George’s School in Newport, Rhode Island, was the starting point for this article.




1. Sidney G. Starling, Electricity and Magnetism for Advanced Students, second edition (Longmans, Green and Co., London, 1916) p. 319.

2. Henry M. Noad, The Student’s Textbook of Electricity, revised by W. H. Preece (Crosby Lookwood and Co, London, 1879) p. 361.

3. E. Taylor Jones, The Induction Coil – Theory and Applications.  This book, published in 1932 and reprinted in 2008, is available on-line and will satisfy those who wish to see a more complete theory of the induction coil.

4. Readers of the 1870 Jules Verne novel, Twenty Thousand Leagues Under the Sea will remember that Captain Nemo took his passengers on extra-vehicular trips with a Ruhmkorff Coil supplying the lamps.  The latter were certainly Geissler tubes, evacuated tubes with a small amount of residual gas that glowed when the high voltage from the coil was applied to its two electrodes.

5. Daniel Davis, Jr., Davis’s Manual of Magnetism (Daniel Davis, Jr., Boston, 1842).

6. Thomas B. Greenslade, Jr., “Charles Grafton Page and His Shocking Coil”, eRittenhouse, 24, No. 1.

7. David Pantalony, Richard J. Kremer and Francis J. Manasek, Study, Measure, Experiment (Terra Nova Press, Norwich Vermont, 2005), pp. 160-162.

8, Robert T. Lagemann, The Garland Collection of Classical Physics Apparatus at Vanderbilt University (Folio Publishers, Nashville, Tennessee, 1983), p. 132.