In the month of April, 1848, Hjorth made a petition to the government for a subvention of 200 rixdollars, in order that he might go to England to get an electro-magnetic machine[1] designed by him, made there. Hjorth had noticed that, in a piece of iron encircled by an electric current, the magnetism only to a certain extent would increase, with the strength of current, there being a point of saturation for the iron. When this point has been reached, it will be of no use to increase the intensity of the current, since the magnetism will not increase any further. On the basis of this observation, Hjorth had constructed his machine, but he had confided the details thereof only to Professors Ørsted and Forchhammer. In the report on Hjorth’s petition made by these two professors to the Board of Trade, they, curiously enough, take exception to the above-mentioned observation by Hjorth, while its correctness will now be acknowledged by any electrician. These professors, however, advised that Hjorth’s petition should be granted, using this liberal argument, worded by Ørsted: »Regarding the petitioner’s new electro-magnetic machine, we must state that we find it quite ingenious, and although we are not convinced that it will produce remarkable effects, we should consider it useful to have a working model executed. Having during so many years worked for this case, the petitioner might perhaps, by the execution of such a model, be enabled to make some further invention, which would bring him nearer to the goal. Indefatigable zeal has often accomplished its purpose, where science had to declare the means at first used, to be entirely inadequate, but where, by continued work, entirely different means, previously unknown to the inventor, were found. Inasmuch as the sum of 200 rixdollars asked for is so small, we find it advisable to grant the subvention. Still we cannot refrain from remarking that the petitioner’s machine may just as well be made here as at any other place«.
Thus the discoverer of electro-magnetism cleared the road which was to lead to the most beautiful application of electro-magnetism, that application which, before all others, has been of radically reforming importance during the last half century, thereby throwing double splendor on Ørsted’s name.
Soon after his arrival at London, in the summer of 1848, through a firm which he knew from an earlier period of his life, Hjorth made the acquaintance of a nephew of Bramah, the renowned mechanician and inventor of the Bramah-lock. Hjorth’s invention was then laid before a friend of Bramah’s, a civil engineer named Gregory, who had made the study of magnetism his specialty. Gregory at once persuaded Bramah to bear the expense of making a machine, and of securing patents in England and several other countries, on condition that the expected profits should be divided between him and Hjorth. Later on, B. Taylor and Normann Innis were taken in as partners, paying together £1000, and then Charles Stovin (£600) and Robert Broad, of the Henley Iron Works (£500). Two machines were now made, according to Hjorth’s directions, by the firm of Robinson & Sons, Pimlico, London. One of these is shown in [Fig. 1], and is apparently quite an ingenious imitation of the steam-engines of those days. C is a movable, A a fixed electro-magnet. Their peculiar shape, involving several conical pins fitting into corresponding cavities, was thought to be advantageous for the distribution of the effect of the magnetic force over a longer stroke. The »piston« C, reciprocating up and down, drives a crank shaft having two opposite cranks. To either of the cranks there is a corresponding group of magnets. An eccentric fixed on the shaft, moves a »slide valve«, alternately closing the circuit of one or the other of the two groups of magnets. When the one piston is at its lowest position, the circuit of the other group of magnets is closed, and its piston is attracted, until it reaches its bottom position; then the current is shifted, and the other piston attracted, etc. In order to avoid the formation of sparks at the circuit breaker, an ingenious device was provided, closing the current of one group of magnets, immediately before that of the other one was broken. The first machine was made with a 4 inch stroke, the next one with 13 1/2 inch stroke. The magnetic attraction per square inch of the piston, had about the same magnitude as the pressure per square inch in the low pressure steam-engines of those days. The patent application was filed in London as early as in October 1848, and it was granted on the 26th of April 1849[2]. On the 21st of September, the same year, Hjorth obtained a fifteen year monopoly in the kingdom of Denmark, to manufacture machines, utilizing electro-magnetism as motive power in the above described manner.
[Fig. 1. Hjorth’s Electromotor.]
The larger of the machines here referred to was shown in action to several technical experts, and created considerable sensation, especially on account of the great length of stroke attained—13 1/2 inches—and the uniform motion of the machine. The machine is mentioned in »Mining Journal«, for the 5th of May, and 16th of June 1849, and an extract of these articles is published in the »Flyveposten« for the 3rd of July the same year.
Hjorth was invited to show the machine at the Royal Society, and at the annual meeting of the Society of Civil Engineers, of which he was a member. It was exhibited at the Universal Exhibition in London, in 1851. In the catalogue it was highly commended, and it received the only prize-medal awarded to electrical machines.
There was, however, one essential obstacle to the practical use of this machine, namely the lack of means for cheaply producing electricity in the quantities required by the electromotor. Wet batteries were expensive to use, and if the machine were to become useful in practice, a powerful »dry battery« would be a necessity. Most of the then known machines producing electricity, were fitted with permanent steel magnets, and as the point of magnetic saturation of steel is low, these machines were unable to produce any considerable quantities of electric energy. Hjorth therefore imposed upon himself the task of building a dry battery. His sketch-book from 1851 is full of new schemes for such batteries and improvements on those already existing (Woolrich’s, Elkington’s and Paine’s). From this it appears, among other things, that he was fully aware that, when the spools suddenly entered or left the field, difficulties would arise in the commutation, and he therefore improved the machine by bending the field magnets, obtaining thereby a gradually increasing and decreasing field, the same thing which is, nowadays, attained by using pointed or obliquely cut pole-shoes.—It would be very tempting to study more closely these sketch-books with their neatly colored drawings, showing how many different ideas have been fostered by him, before the actual production of the first dynamo, in 1854. Most of the descriptions and notes have been written in the English language, which he used almost as readily as his native tongue. On the 1st of May, 1851, Hjorth writes in his sketch-book, beside a sketch of a machine having copper discs for armature conductors and cast iron electro-magnets: »By passing the current on the said way round the Electromagnets, these will of course be excited in proportion to the strength of the same, and the more they are excited, the more will the discs be influenced by the magnets, a mutual action thus taking place«.
So it appears that Hjorth, as early as on the 1st of May 1851, with perfect clearness, has pronounced the dynamo-electric principle.
Under the date of June 24th, 1851, we find sketched out another beautiful idea for the construction of a dynamo. It must be regretted that this machine has not been executed, as it would certainly have proved superior to his dynamo of 1855, which has many points in common with this project. [Fig. 2] shows a reproduction of this page of the sketch-book. There is no descriptive text to this sketch, only at one side of the drawing, these very significant words are written: »Magneto-Electric arrangement with mutual action«. All the six powerful held magnets are of cast iron, and they are wound so as to be magnetized by the current, produced by the dynamo itself[3].