WILLIAM DU BOIS DUDDELL Biography - Theater, Opera and Movie personalities

 
 

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WILLIAM DU BOIS DUDDELL
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William du Bois Duddell was an English electrical engineer, the inventor of the electromagnetic oscillograph and some other electronic instruments. However, he is more known as the inventor of the ‘Singing Arc’ intended to provide both light and music by means of a tunable arc of electricity. This was one of the first electronic musical device.

       

William du Bois Duddell was an English electrical engineer. He was born in 1872. William du Bois Duddell invented many important electronic instruments including the moving coil oscillograph (an early device for the photographic recording and observation of oscillating audio frequency waveforms which was still in use in the 1940s), thermo-ammeter, thermo-galvanometer (an instrument for measuring extremely small currents and potential differences) and a magnetic standard, which was used for the calibration of ballistic galvanometers. He also built an alternator giving 120,000 cycles per second.

       

Oscillograph is the instrument for indicating and recording time-varying electrical quantities, such as current and voltage. The two basic forms of the instrument in common use are the electromagnetic oscillograph and the cathode-ray oscillograph; the latter is also known as a cathode-ray oscilloscope, which, strictly speaking, is purely an indicating instrument, while the oscillograph can make permanent records.

       

The operation of an electromagnetic oscillograph, like the operation of a d’Arsonval galvanometer, depends on the interaction of the field of a permanent magnet and a coil of wire through which an electric current is flowing. Some oscillographs were provided with a pen arm, attached to the coil, that traces an ink record on a moving paper chart. The most common device of this nature was the electrocardiograph, which employs a coil of fine wire with many turns and is used for studying heart action.

       

The light-beam oscillograph has much less weight to move than does the pen-writing instrument and so responds satisfactorily to higher frequencies, about 500 Hz, or cycles per second, compared with 100 Hz for the pen assembly. It uses a coil to which a small mirror is attached. A beam of light is reflected from the mirror onto a photographic film moving at a constant speed.

       

In Duddell’s oscillograph the galvanometer part consists of an electromagnet in the field of which is stretched, a loop of very fine wire. To this is attached a mirror; hence, if a current goes up one side of a loop and down another, the wires are oppositely displaced in the field.

       

The loop and mirror move in a cavity full of oil to render the system dead-beat. A ray of light is reflected from this mirror and from another mirror which is rocked by a small motor driven off the same circuit, so that the ray has two vibratory motions imparted to it at right angles, one a simple harmonic motion and the other a motion imitating the variation of the current or electromotive force under test.

       

This ray can be received on a screen or photographic plate, and thus the wave form of the current is recorded. In the Duddell’s oscillograph it is usual to place a pair of loops in the magnetic field, each with its own mirror, so that a pair of curves can be delineated at the same time, -and if there is any difference in phase between them, it will be detected. Thus we can take two curves, one showing the potential difference at the end of an inductive circuit, and the other the current flowing through the circuit. The Duddell’s oscillograph allowed the photographic recording and observation of oscillating audio frequency waveforms.

       

The outfit illustrated in these pictures has been developed by the Cambridge Instrument Company, Ltd., based on the Duddell’s general scheme to meet the demand for an instrument of this type which may be used on circuits up to 100,000 volts. The outstanding advantage of the design, as compared with that of earlier models, lies in the fact that simultaneous records may be obtained from three vibrators which may be any combination of the electromagnetic and the electrostatic types-only one camera and one source of light being required.

       

The source of light is an ordinary 6-volt metal filament lamp to which a greatly abnormal voltage is applied for a fraction of a second during the time of exposure. The intense light thus obtained passes through an optical system which ensures that a separate beam is focussed on each vibrating mirror. The reflected beams pass through a cylindrical lens, thereby reducing the images to bright spots of light on the photographic paper, or film, which is traversed in a vertical plane at right angles to the plane of the vibrating beams.

       

The complete outfit was enclosed in a compact metal case, which is approximately 90 cm. in length, 31 cm. in width and 36 cm. high. The oscillograph was used to study the recurrent or transient phenomena of almost any commercial or laboratory circuit. It was normally suitable for use on commercial circuits up to 600 volts.

       

Typical oscillograms are reproduced, approximately three-quarters actual size.

       

Duddell’s Thermo-Ammeter
manufactured by the Cambridge Scientific Instrument Company, Ltd., England, 1908

       

The general appearance of the instrument is shown in the figure below, while the figure above is a sectional diagram showing the general arrangements of the heater and coil, etc. D is the moving coil which moves in the field produced by the permanent magnet BB. AA are soft iron pole pieces and C is a cylindrical core so hat the field in which the coil moves is truly radial.

       

EE are the pivots which it will be seen are fixed inside the coil so that when the instrument is in a horizontal position (the correct position for use), the coil is practically suspended from the top pivot, the lower pivot being almost entirely out of action. By this means pivot friction is reduced to a minimum. The ends of the coil are brought out at the bottom and soldered to the ends of the thermo-junction LM, the elements of which are made from special allows which have a very high thermo-electric force. The lower ends of the couple are soldered to a thin circular “receiving plate".

       

Immediately below the receiving plate the heater K is fixed. In instruments to give the full deflection for 20 mA or less, this heater consists of a sheet of platinized mica, the platinum being scraped away to form a sort of gridiron. By this means resistances of several hundred ohms may be easily obtained in a space of less than 0.2 cm2. For currents above 20 mA the heater is of wire.

       

When in use the current to be measured passes through the heater K, which in consequence becomes heated and so warms the receiving plate J and thermo-couple LM. The resulting E.M.F. of the couple causes a current to flow round the coil which turns in the magnetic field, the deflection being indicated by a pointer moving over a scale in the usual way. It will be seen that no current passes through the control spring F; the material for the spring may therefore be selected without reference to its electrical resistance. One end of the control spring is attached to the lever G, which is connected to the zero adjusting screw outside the case. The whole instrument is contained in a polished teak case with a leather handle.

       

The Duddell’s Thermo-Galvanometer
manufuctured by the Cambridge Scientific Instrument Company, Ltd., England, 1905

       

A single loop of silver wire L is suspended by means of a quartz fibre Q, between the pole pieces NS of a permanent magnet. The loop is surmounted by a glass stem G carried by a mirror M, whilst its lower ends are connected to a bismuth antimony thermo-couple (Bi, Sb). The heating resistance or “heater", consisting of a fine filament of high specific resistance (usually a platinized quartz fibre) is fixed immediately under the thermo-couple. One end of the heater is connected to the frame of the instrument to avoid electrostatic forces. Part of the heat due to the passage of the current through the resistance is radiated and carried by convection on to the thermo-junction and the resulting E.M.F. applied to the ends of the loop causes it to turn in the magnetic field. The heater filament is straight and only 3 or 4 mm long and therefore forms, with the two straight wires leading to the terminals of the instrument, a single narrow loop of exceedingly small self-induction or capacity.

       

The general appearance of the instrument with case removed is shown here. The suspension, heater, etc., are protected from sudden changes in temperature by being enclosed in a heavy metal block the front E of which is removable. This front (shown removed) slides over the studs DD and is held in place by two milled heads. The heaters are set up in small protecting cases with contact rings so that they can be interchanged quickly when it is desired to greatly alter the sensitivity of the instrument.

       

The sensitivity may also be altered by altering the distance between the couple and the heater. This adjustment is made by means of the ebonite milled head F. The clip which holds the heater has a spherical seating so that it may be adjusted centrally under the thermo-couple by means of the set screws G. The suspension may be securely clamped by unscrewing the pin B. A stout mahogany cover (not shown in the illustration) protects the instrument from dust and heat radiation.

       

The Duddell’s Thermo-Galvanometer

       

Duddell’s magnetic standard
manufactured by the Cambridge Scientific Instrument Co. Ltd., England, 1912

       

The Duddell magnetic standard was used for the calibration of ballistic galvanometers in conditions similar to those in which they were used. A known direct current of up to 10 A is passed through the fixed primary coils, and the galvanometer is connected in series with the secondary coils.

       

Releasing the lever on the front allows the secondary coils to reverse, causing a known flux change through the secondaries, and thereby a known total charge to flow through the galvanometer. The deflection of the ballistic galvanometer is proportional to the total charge, and therefore this allows it to be calibrated. The two primary coils are wound in opposite directions to negate the effect of any stray magnetic fields, and the change in current due to heating is negligible The flux change per ampere of current through the primary coils therefore cannot be changed except by physical damage to the instrument.

       

However, William du Bois Duddell is more remembered because of his invention of one of the first electronic musical instruments: ‘Singing Arc’.

       

A carbon arc lamp

Before Thomas Alva Edison invented the electric light bulb electric street lighting was in wide use in Europe. A carbon arc lamp provided light by creating a spark between two carbon nodes. The problem with this method of lighting, apart from the dullness of the light and inneficient use of electricity was a constant humming noise from the arc. The British physicist William Duddell was appointed to solve the problem in London in 1899 and during his experiments found that by varying the voltage supplied to the lamps he could create controllable audible frequencies. Demonstrated in London in 1899, Duddell’s instrument was controlled by a keyboard, which enabled the player to change the arc’s rate of pulsation, thereby producing distinct musical notes.

       

The rate of pulsation of an exposed electric arc was determined by a resonant circuit consisting of an inductor and a capacitor. This was the first electronic instrument that was audible without using the telephone system as an amplifier/speaker.

       

When Duddell exhibited his invention to the London institution of Electrical Engineers it was noticed that arc lamps on the same circuit in other buildings also played music from Duddell’s machine this generated speculation that music deliverd over the lighting network could be created. Duddell didn’t capitalise on his discovery and didn’t even file a patent for his instrument.

       

Duddell toured the country with his invention which unfortunately never became more than a novelty. It was later recognised that if an antena was attached to the singing arc and made to ’sing’ at radio frequencies rather than audio it could be used a continuous radio wave transmitter.

       

The method of generating continuous electric waves by means of a arc lamp was invented by the Danish engineer Valdemar Poulsen in 1902 as a modification of William Du Bois Duddell’s ’singing arc’ of 1900. The carbon arc lamp’s audio capabilities was also used by Thadeus Cahill during his public demonstrations of his Telharmonium ten years later.

       

William du Bois Duddell was President of the IEE in 1913 and a trustee of the Benevolent Fund. The Duddell medal was instituted in 1923 as a memorial to William du Bois Duddell. It is awarded annually to a person who has contributed to the advancement of knowledge through the application of physics, including invention or design of scientific instruments or by the discovery of materials used in their construction or has made outstanding contributions to the application of physics. The bronze medal is accompanied by a certificate and a prize of £1000.


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