In the library of the Royal Society, Davy’s notes made during the years of 1805 and 1812 are available in two large volumes.  These were arranged and paged by Faraday, who was destined to contribute greatly to the future development of the science and art of electricity.  In one of these volumes is found an account of a lecture-experiment by Davy which certainly is a description of the electric arc.  An extract of this account is as follows: 

The spark [presumably the arc], the light of which was so intense as to resemble that of the sun, ... produced a discharge through heated air nearly three inches in length, and of a dazzling splendor.  Several bodies which had not been fused before were fused by this flame....  Charcoal was made to evaporate, and plumbago appeared to fuse in vacuo.  Charcoal was ignited to intense whiteness by it in oxymuriatic acid, and volatilized by it, but without being decomposed.

From a consideration of his source of electricity, a voltaic pile of two thousand plates, it is certain that this could not have been an electric spark.  Later in his notes Davy continued: 

...the charcoal became ignited to whitness, and by withdrawing the points from each other, a constant discharge took place through the heated air, in a space at least equal to four inches, producing a most brilliant ascending arch of light, broad and conical in form in the middle.

This is surely a description of the electric arc.  Apparently the electrodes were in a horizontal position and the arc therefore was horizontal.  Owing to the rise of the heated air, the arc tended to rise in the form of an arch.  From this appearance the term “arc” evolved and Davy himself in 1820 definitely named the electric flame, the “arc.”  This name was continued in use even after the two carbons were arranged in a vertical co-axial position and the arc no more “arched.”  An interesting scientific event of 1820 was the discovery by Arago and by Davy independently that the arc could be deflected by a magnet and that it was similar to a wire carrying current in that there was a magnetic field around it.  This has been taken advantage of in certain modern arc-lamps in which inclined carbons are used.  In these arcs a magnet keeps the arc in place, for without the magnet the arc would tend to climb up the carbons and go out.

In 1838 Gassiot made the discovery that the temperature of the positive electrode of an electric arc is much greater than that of the negative electrode.  This is explained in electronic theory by the bombardment of the positive electrode by negative electrons or corpuscles of electricity.  This temperature-difference was later taken into account in designing direct-current arc-lamps, for inasmuch as most of the light from an ordinary arc is emitted by the end of the positive electrode, this was placed above the negative electrode.  In this manner most of the light from the arc is directed