Valdemar Poulsen was Danish telephone engineer and inventor, best known for his Telegraphone, which he patented in 1898. It was the first practical apparatus for magnetic sound recording and reproduction. It recorded, on a wire, the varying magnetic fields produced by a sound. The magnetized wire could then be used to play back the sound.
He also invented the Poulsen Arc Transmitter – the first device for generating continuous radio waves, thus aiding the development of radio broadcasting. Valdemar Poulsen, son of a Danish High Court judge was born 23rd November 1869 in Copenhagen. Valdemar was not a good scholar; the only subjects he was interested in were physics and drawing.
He had no interest in mathematics, a trait he shared with many other great inventors. His father wanted him to become a doctor but after an unsuccessful time at medical school at the University of Copenhagen, 1889-93, at the age of 24, he obtained a position in the technical section of the Copenhagen Telephone Company. His work was mainly troubleshooting which allowed him a fair amount of time for experimenting.
While working there, Poulsen became interested in magnetic recording of sound. There seems to be no record of what gave Poulsen the idea of magnetising steel wire to make sound recordings. Maybe he had read an article written in 1888 by American scientist Oberlin Smith for the magazine Electrical World.
In his article Smith discussed the possibility of permanent magnetic impressions for recording sound and suggested, as a medium, cotton or silk thread, in which steel dust was suspended. Smith also considered steel wire but didn’t think it would be possible ‘that it would divide itself up properly into a number of short magnets” to establish a magnetic pattern as a replica of currents produced by a microphone. Smith never built a machine or proved his theory practically.
In an early experiment Poulsen stretched a steel wire between two parallel walls, inclined at such an angle that a small electromagnet suitably attached to the wire could, assisted by gravity, slide down the wire at a uniform speed. Wires attached to the electromagnet energised it from the voltage of a battery modulated by a microphone. For replay the battery was disconnected, and the microphone replaced by a telephone earpiece, the electromagnet returned to the top and let go. The experiment worked and Poulsen set about putting magnetic recording to use in the shape of a telephone answering machine.
On 1st December 1898, he filed a patent in Denmark for the Telegraphone (or in Danish, Telegrafoon), the first device in history to use magnetic sound recording. An extract from this patent reads: “The invention based upon the fact that when a body made of magnetisable material is touched at different points and at different times by an electromagnet included in a telephonic or telegraphic circuit, its parts are subject to such varied magnetic influences that conversely by the action of the magnetisable body upon the electromagnet the same sounds or signals are subsequently given out in the telephone or recording instrument as those which previously caused the magnetic action upon the magnetisable body.”
Drawing from the Poulsen’s Telegraphone Patent of 1898
That the idea of recording sound magnetically was Poulsen’s alone is proven by the fact that nobody else has ever claimed credit for the invention, unlike the situation surrounding most other electrical devices such as the telephone, phonograph, cinema, sound-on-film and television. Wire recording, tape recording, hard disk, floppy disk, credit cards and train tickets – Poulsen had them all covered as shown in the following extract from his 1899, UK Patent No 8961.
“Instead of a cylinder with a helical steel wire there may be uses as a receiving device a steel band, supported if necessary on an insulating material and brought under the action of an electromagnet. Such an arrangement has the advantage that a steel band of an desired length may be used.
Instead of a cylinder there may be used a disk of magnetisable material over which the electromagnet may be conducted spirally; or a sheet or strip of some insulating material such as paper may be cover with a magnetisable metallic dust and may be used as the magnetisable surface. With the aid of such a strip which may be folded, a message received at any place provided with the new apparatus may be sent to another place where it may be repeated by passing the strip through the apparatus at that place.”
The original Telegraphone consisted of a spirally grooved brass cylinder around which, embedded in the groove, was wound a .01″ diameter steel wire. The two poles of an electromagnet, energised by the voice currents from the microphone, rested against the wire wound round the cylinder.
The cylinder remained stationary while the electromagnet rotated around the coil, the wire being magnetised by amounts corresponding to the strength of the voice currents. When the recording was complete the microphone was switched out of circuit and a telephone receiver connected in its place. The electromagnet now returned to the start and on rotating this time the recorded message was heard back.
Poulsen’s ‘Drum’ Telegraphone, ca. 1898
By the standards then prevailing it worked quite well. Reports by those who heard it remarked on the “naturalness of the reproduction” and the “freedom from noise”. These comparisons were of course based on the quality of acoustically recorded Phonograph cylinders of the period. After inventing the Telegraphone, Poulsen left the telephone company in order to be free to conduct a series of experiments and to follow a new line of investigation that had suggested itself to him.
Patents were taken out throughout the world and Poulsen continued to improve the Telegraphone along with an associate Peder Olaf Pedersen, also an excellent engineer. The next development was a reel-to-reel machine using steel wire passing over a static recording head at 7ft per minute.
Poulsen’s Telegraphone, improved model
Valdemar Poulsen and Peter Jensen with other co-workers at Lyngby, 1907
The Telegraphone received considerable attention when it was exhibited at the Exposition Universelle in Paris in 1900. The few words that the Austrian emperor Francis Joseph spoke into it at that exhibition are believed to be the earliest surviving magnetic recording. Valdemar Poulsen received the Grand Prix of Paris for his Telegraphone. Even with this encouragement, he could not find financial backers in Europe. In 1903, with American associates, Poulsen founded the American Telegraphone Company for the manufacture and sale of an improved version of his device.
The telegraphone recorded continuously for 30 minutes on a length of steel piano-wire moving at a speed of 84 inches (213 cm) per second. Despite the fact that the Telegraphone had a number of advantages over the wax cylinder Dictaphone – better quality, longer playing time, reuse of the media – the company eventually went into receivership. This was probably due more to bad business practice and scandal surrounding the president of the company than to the limitations of the Telegraphone – the low sound output, length of rewind time before replay possible and the wire often tangling. The device did not have wide application, however.
In 1927, American inventor J.A. O’Neill replaced the wire with a magnetically coated ribbon and since then magnetic tape recorders have dominated the recording industry.
Poulsen is also known for his work in improving wireless transmission by means of the arc. In 1903 Poulsen obtained an English patent on his adaptation of a “singing arc” for radio purposes. Invented by the Englishman William Duddell, the singing arc could generate continuous audio waves (hence its name). Poulsen transformed this device so that it could generate radio waves. Poulsen’s arc as a generator of continuous waves differed from the usual arc since it burned in an atmosphere of a hydrocarbon gas in a strong transverse magnetic field.
The Poulsen system is based on the discovery of Mr Duddell that if a current arc is shunted by a circuit containing capacity and inductance there will be established in the circuit electrical oscillations, the frequency of which depends upon the value of the inductance and capacity. The reason for this is that unlike a metallic conductor the arc does not follow Ohm’s law and the curve showing the relation between current and terminal voltage is not a straight rising line, but has what is termed a falling characteristic, that is to say, if the current through the arc be increased the potential difference at its terminals will drop.
Suppose now that a circuit with capacity and inductance in series is placed across the terminals of an arc, the condenser will charge, and in doing so, the current through the arc lessened, the potential difference at its terminals will increase and the condenser to a still higher voltage. After the capacity is fully charged the current through the arc will increase, and owing to the drop in voltage which it causes the condenser will discharge across the arc, and the discharge will, if the resistance is small, be oscillatory.
In order to obtain oscillations of considerable energy Mr. Duddell found that it was necessary to use a capacity of the order of 1 microfarad, and with a capacity of this magnitude it was not possible to obtain the very high frequencies needed for Radio-Telegraphy.
Poulsen’s great discovery was the effect of a hydrogen atmosphere which by cooling the arc increased the steepness of its characteristic curve, and also the use of very powerful magnetic field which enabled him to get a high terminal voltage. By the use of the arc burning in a hydrogen atmosphere, and the powerful transverse magnetic field, he was able to use a small capacity and thus get oscillations of the frequencies useful for Radio-Telegraphy and at the same time powerful.
The practical construction of the Poulsen arc is as follows: the anode is made of copper and the end takes the form of a beak. The cathode is of carbon about one inch in diameter, the arc striking between the copper beak and the edge of the carbon. The carbon is fitted in a holder which is slowly rotated by means of a small motor, and as it burns away a fresh surface is presented and the length of the arc kept constant. The arc-length is also adjustable by means of a screw fitted to the copper electrode.
The electrodes are taken through insulating sleeves in the sides of a water-cooled metallic chamber which is also flanged on the outside to assist the cooling. Through the sides of the chamber, and transversely to the electrodes, pass the pole pieces of a powerful electro-magnet which blows the arc out into a loop, the winding of the magnets being in series with the arc also serve as a choking coils and prevent the oscillations from passing back into the supply circuit. The chamber in which the arc burns is supplied with hydrogen through a tube let into its base and after passing through the chamber escapes through an outlet at the top and is conveyed away by means of a tube connected to it.
The Original Poulsen Arc Converter, 1908
The arc is connected across a 500 volt direct-current supply an across it is shunted the primary circuit, which consists of a condenser and an inductance in series. The antennae is connected to one point of the inductance and the earth wire to another. Signalling is effected by shorting through the Morse key a turn or two of the inductance which alters the wave-length and throws the transmitter in and out of tune with a receiver, a difference of about 5 per cent, being sufficient.
The Poulsen Arc Transmitter
As energy is supplied to the antennae at every swing the oscillations emitted from the Poulsen generator are continuous and undamped, or practically so. The Poulsen transmitter is unlike that of any other system inasmuch as no detector is made use of, but the received energy accumulated in a condenser and discharged at intervals through the telephone by means of a piece of apparatus which the inventor has named a tikker.
A primary coil with variable condenser across its terminals to adjust the turning is coupled to the secondary circuit, which also consists of a coil and a variable condenser; across the terminals of this condenser is joined a mica condenser of fairly large capacity and the tikker, which is an intermittent contact formed by two gold-plated brass wires crossing each other at right angles, one of them being mounted at the end of a small electro-magnetic make and break similar in construction to a trembler bell. The telephone, which is of low resistance, is joined across the mica condenser.
The action of the tuner is as follows: the primary having been tuned to the sending-station and the secondary tuned to the primary; during the intervals when the tikker contacts are open the secondary circuit is left free resonate up, and the energy of many oscillations thus accumulated when the tikker contacts close, the circuit, owing to the added capacity of the mica condenser, which is now in parallel, will oscillate to a lower frequency. The opening of the tikker contacts will be determined by the presence or absence of a current across them, as this determines the conductivity of the small gap between the wires as the tikker starts to open.
It will thus be seen that when the current is passing through zero the mica condenser, charged as it is with the greater part of the energy, will be disconnected from the secondary circuit and discharge through the telephone. The coupling between the primary and the secondary is very loose and full use is thus made of resonance, the tuning being so sharp that a difference of 4 or 5 per cent. In the wave-length is sufficient to render the signals inaudible. The tikker method, although one of the most sensitive means known for detecting electrical oscillations, labours under the disadvantage that it is not able to receive signals from the ordinary spark transmitters which give out damped and discontinuous oscillations.
In 1903 Poulsen developed an arc transmitter which increased the frequency range of Duddell’s Singing Arc (1900) from 10 kHz to 100 kHz, enabling speech to be transmitted up to a radius of 150 miles. By 1920 the Poulsen Arc transmitter was as powerful as 1000 kW with ranges of up to 2,500 miles.
(See additional information on the Poulsen’s transmitters)
Valdemar Poulsen with his speech transmitter at Lyngby, Denmark in 1908
The Federal Telegraph Company, specializing in arc transmitters, brought Poulsen’s arc to America. When NAA, the United States naval spark station at Arlington, Virginia, went into commission in 1912, an arc also was installed; thus two rivals, Fessenden with the spark, Poulsen with the arc met on a common proving ground. Arc transmitters up to 500 kilowatts were tested by the Navy.
One main disadvantage was found in that the arc emitted harmonics and arc mush; the heat was so terrific that a water cooling system was required. Nevertheless, during the First World War a number of battleships carried arc transmitters.
The First Complete Arc Transmitter and receiver built by Poulsen Wireless in 1910 in Palo Alto (left to right: Doug Perham, F. Albertus, and Peter V. Jensen). Jensen left shortly after this photo was taken to start the Magnavox [loudspeaker] Co.
Federal Telegraph’s engineering staff, 1917 (left to right: Leonard Fuller, Chief Engineer; Harold Elliot, Chief Draftsman; Corwin Chapmen, Lab. Engineer; Kurt Blew, Shop Forman; Ralph Beal, Assistant Chief Engineer; and Adrien Anderson, Factory Engineer). The original Poulsen 100 W arc, brought from Denmark by Elwell, is in the forground. The first of six 500 kW arcs, built for the U.S. Navy, stands behind the group. It was eventually installed at Pearl Harbor, Hawaii.
Later the U.S.S. George Washington, which took President Wilson to the Peace Conference, was equipped with an arc in hopes that communication might be maintained all the way across the sea. It was a triumph for radio when the Washington entering the harbor at Brest flashed signals from its arc which were picked up at Otter Cliffs, Bar Harbor, Maine, and a 600-word message was received without the loss of a word.
Subsequent efforts with this device by Poulsen and others made long-wave radio broadcasting possible by 1920.
1000 kw Poulsen Arc Converter installed at Lafayette Radio Station
Although Poulsen never graduated from university we was well acclaimed by his fellow inventors and scientists. In 1907 Poulsen received the Gold Medal of the Royal Danish Society for Science, and in 1909 the University of Leipzig conferred upon him the honorary degree, Doctor of Philosophy. He received from the Danish government the Medal of Merit, an honor he shared at that time with Nansen, Georg Brandes, Sven Hedin and Amundsen. At his death Dr. Poulsen was a fellow of the Danish Academy of Sciences, the Danish Academy of Technical Science and the Swedish Institute for Engineering Research, and an Honorary Doctor of the University of Leipzig.
Valdemar Poulsen died in 1942. The picture shows his grave.
Danish postal stamp memorizing 100th Birthday of the Inventor Valdemar Poulsen.