039 DOLEZALEK QUADRANT ELECTROMETER (c1903)
Cambridge Scientific Instrument Co., Cambridge, England. S/N 16979
30*16*16
A light, two bladed aluminium vane is suspended in the center of quadrant shaped boxes, the opposite pairs of which are connected to the same potential on terminals under the base. The vane is attached to a light rod which carries a small mirror suspended by a thin quartz fibre (missing) from the torsion head. A brass case with window opposite the mirror completes the apparatus.

The quadrant electrometer may be used to compare the EMF of two cells; verify Ohm's Law; measure a high resistance; compare large and small capacitances and determine the dielectric constant. This is done by deflection of the aluminium vane via a difference in the electric potential of the pairs of quadrants. The angle of deflection is measured by shining a lamp on the mirror, which reflects the light onto a scale placed a certain distance away. The instrument must be calibrated so that the scale used is accurate and its sensitivity known but it draws no current from the circuit and is sensitive to a fraction of a volt.
References:

  1. Worsnop,B.L. and Flint, H.T., (1962) Advanced Practical Physics for Students, Methuen & Co., London, pg 643-62
  2. Cambridge Electrical Instruments. for D.C. Measurements Booklet No.III pg 68.
  3. K.Lyall, The Whipple Museum of the History of Science Catalogue 8
  4. ELECTRICAL AND MAGNETIC INSTRUMENTS, item199.

B.S.

058 FIBRE ELECTROMETER (c1923)

E. Leybold's Nachfolger A.G., Cöln - Rhein, S/N.

5* 4* 8

An electrometer is a device used to quantitatively measure differences in electrical potential.

This instrument is also referred to as a String Electrometer. This type of electrometer was originally designed by Horace Darwin and Thomas Laby and was produced by the Cambridge Instrument Company.

There are many different types of electrometers. In this particular design, a fine silvered fibre is stretched between two vertically aligned metallic triangular beams. The fibre is also aligned vertically between the beams. The tension applied to the fibre is altered using the micrometer adjustment on the top of the device.

The metallic beams are arranged so that the peaks of the triangles are pointing inwards to the fibre. Electrical potentials are applied to two external electrodes on the device. These electrodes are connected to the metallic beams via two 0.5 megohm resistors, one per beam. The horizontal position of each beam is altered using micrometer adjustments on either side of the device. These move the metallic beams closer or further from the fibre.

When different electrical potentials are applied to the two plates, the fibre is displaced. This displacement can be observed through a microscope. The position and focus of this microscope can be adjusted using fittings on the front of the device. The observation microscope has a scale marked on it so quantitative measurements of fibre deflection can be made. To observe the fibre, a light source is required at the back of the device. A mirror is attached for this purpose. However, any light source could be used.

The principles behind the operation of this device are similar to those of an electroscope. The fibre which has a neutral charge, is attracted to both of the two beams. However, the fibre will be more strongly attracted if one beam has a higher potential or is significantly closer to the fibre. This is in accordance with Coulomb's Law which can be stated as:

The magnitude of the electrical force that a particle exerts on another particle is directly proportional to the product of their charges and inversely proportional to the square of the distance between them.

Thus, by adjusting the horizontal position of either beam in relation to the fibre, the effect of the potential applied to that beam can be observed on the altering deflection of the fibre. Similarly, altering the electrical potential on either beam will alter the deflection of the fibre. If the fibre is not deflected to either the right or the left, the forces generated by each beam are equal at the position of the fibre. If the two beams are equally spaced from the fibre, the potentials applied to each beam should be the same.

References:

  1. British Optical Instrument Manufacturers' Association; Dictionary of British Scientific Instruments; Constable and Company Ltd., London, 1921.
  2. Muirhead, Ed; A Man Ahead of his Times - T.H. Laby's Contributions to Australian Science; Spectrum Publications Pty. Ltd., Richmond Victoria, 1996.
  3. Cattermole, M.J.G., Wolfe, A.F.; Horace Darwin's Shop - A History of the Cambridge Scientific Instrument Company 1878 To 1968; IOP Publishing Limited, Bristol England, 1987.
  4. Ohanian, Hans C.; Physics - Volume 2, 2nd Edition; W.W. Norton and Company, New York, 1989.

MM

 

122 DUDDELL MAGNETIC STANDARD (1912)
The Cambridge Scientific Instrument Co. Ltd./Cambridge, England No. 15104 B185
30*14*11
Wooden case with brass front and lever with two spring catches which hold it in one of two orientations. Lever reverses two secondary coils inside two fixed primaries. The secondary coils are spring loaded, the springs also carrying the current for the secondaries.

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 10A 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.

References:

  1. Cambridge Electrical Instruments for D.C. Measurements Booklet No. 111, p.37.

D.B.

129 POTENTIOMETER
The Cambridge Scientific Instrument Co. Ltd., Cambridge, England No. 17139 (1913)
47*24*12 cm
Consists of a wooden case with an ebonite top. A wooden frame mounted on top contains eight copper bridges and frames, eight pairs of small black, plastic, cylindrical pots, each containing a resistance contact. White lettering on the front of the frame below every pair of pots, labels then 10, 20, 40, ... to 1280. Four pairs of brass, screw-top terminals are behind the frame, labelled G, P, C and B from left to right. A slidewire variable resistance, consisting of two wires, with a vernier scale is in front of the frame. The large scale of the slidewire resistance is silvered and engraved 0-30cm, with 1mm divisions and the vernier scale is labelled 0-10. The slidewire has a black, plastic slide, with a brass lock to keep the slide in position. No units are marked on any scales.

This instrument was used to measure the potential of a cell by comparing it to a standard cell, or to measure the potential difference between two cells, with reference to the standard. The potentiometer provides the variable resistance required for this and is used in conjunction with a galvanometer, to note when zero current flows and the variable resistance is correctly adjusted.

The galvanometer is connected to the terminals labelled G and the standard cell across B. Cell to be compared is connected across those labelled P and if a second cell is also to be used, it is connected across C. The pots containing the contacts also contained mercury. To reduce the resistance, the copper bridges are moved from their positions in the frame and placed to connect a pair of contacts. These bridges and the mercury pool contacts from a low-resistance path for the current and short circuit the desired resistance (resistances in 0.1 ohms, although not labelled so, i.e. 1280 refers to 128.0 ohms), connected below and across the two contacts, inside the case. The slidewire variable resistance is available for fine adjustment of the resistance. Each slidewire has a total resistance of 1.5 ohms, hence the 0-30 cm scale can be converted to a 0-3 ohm scale, providing accuracy to two decimal places.

When the galvanometer detects that there is no current flowing, i.e. the resistance has been correctly adjusted, the potentials of the cells may be calculated with reference to the standard cell, according to the division of the variable resistance.

Some of the advantages to this method of determining potentials is that there is no need to measure the deflection of the galvanometer (require it to read zero), the vernier scale provides considerable accuracy and the measurements are taken with no current flowing through the cell, therefore there is no need to consider the effects of the internal resistance.

References:

  1. Duncan, G. and Starling, S.G., Text-book of Physics, Macmillan and Co. Ltd., 1944, p.890,891.

L.D.

201 POTENTIOMETER
Otto Wolff, Berlin, 1914
51*32*23
Mahogany case has ebonite plate carrying heavy brass contacts and switches with ebonite handles. Inside the case are resistance coils, calibrated at 20C.

Such potentiometers were used to measure voltages by comparison using a battery as energy source, a standard cell as voltage reference, and a galvanometer . The galvanometer was connected between the point of unknown potential and one terminal of the potentiometer. When the two are at the same potential, no current flows. The voltage at the potentiometer terminal can be varied in a controlled way by switching in combinations of resistance coils. A similar measurement with a standard cell of accurately known voltage calibrates the result. At "balance" no current was drawn from the circuit under test, which is a great advantage, but , although the method is very accurate, it was rather slow.

References:

  1. B.L.Worsnop and H.T.Flint, Advanced Practical Physics for Students, Methuen and Co.,,London, Ninth Ed.,1957, Ch XXI

N.H.

166 MILLI-VOLTMETER(Weston Direct Reading Lab. Standard) (1890)
Weston Electrical Instrument Co./Newark, N.J. U.S.A.Model No 972, PLUQ E137
39*35*14
Portable, pivoted-coil DC Milli-Voltmeter; black painted metal case on wooden base; 10-40C thermometer and a level indicator are also mounted on the wooden base; Glass window showing mirror and white dial.

The millli-voltmeter contains a coil of thin copper wire that rotates on an axis, that is pivoted on jewelled pivots, and is perpendicular to the magnetic field of a permanent magnet. It is the displacement of this coil, when a current flows through the coil, against a mechanical restoring force that indicates the presence polarity and strength of a current. There is a soft iron cylinder (strengthening the field and making it approximately radial in the space inside the coi) fixed to a brass bar which forms a bridge between the pole pieces. Hair springs are attached to the axis of the pivoted moving coil, where they establish a restoring couple, and to points on the framework insulated from one another. They also form leads for the current to and from the coil. Attached near the upper spring is a light counterpoised pointer which moves over a scale. There is a resistance in series with the coil to avoid errors due to rearrangement of current and potential drop in the circuit. Voltage is indicated by a mechanical displacement of a pointer against a scale. To use the instrument it must be placed in parallel with the points whose potential difference is to be measured. Weston's design remains basically unchanged up to the present day.

References:

  1. B.Worsnop and H.Flint, Advanced Practical Physics for Students, Methuen, London, (1950), pg 493. J.T.Stock and D.Vaughan, The Development of Instruments to Measure Electric Current, Science Museum, London (1983), pg 27.

A.B.

118 SUMPNER REFLECTING ELECTRODYNAMOMETER
Robt. W. Paul, London,N. No. 31.
23*18D
A circular ebonite base with three levelling feet carries a reflecting galvanometer with its coil suspended between the poles of an electromagnet. The suspension also carries a mirror to reflect a beam of light to a scale. The cylindrical brass cover has a window to pass the beam.

Since the deflection of the mirror depends on the product of the currents flowing in the coil and in the electromagnet windings the instrument can be used not only for measurements of current but also for electrical power, by making the current through the coil proportional to the voltage across the load.. This applies equally well to AC as to DC circuits as the device accounts for any phase difference between the two inputs. Thomas Parnell, who later became the first Professor of Physics at UQ, used this instrument in work on AC circuit measurements which was reported in the Physics Department's first research publication in 1917. His paper showed how the device could be used as a sort of phase-sensitive detector to separate the resistive (in phase) and reactive (out of phase) components of the impedance of an inductor under test.

References:

  1. T.Parnell, Proc. Phys. Soc. London, XXIX, pg 259-268 (1916-1917)
  2. K.Lyall, The Whipple Museum of the History of Science, Cat.8, Electrical and Magnetic Instruments, item 164.
  3. J.Stock and D.Vaughan, The Development of Instruments to Measure Electric Current, Science Museum 1983, pg 39

N.H.

016

037 MULTICELLULAR VOLTMETER (c1899)
Lord Kelvin's Patents Multicellular Voltmeter
James White, Glasgow, No. 1405
34*20*20
Cylindrical brass case, mounted on square wooden base with three levelling screws. Circular 20 cm diameter bevelled glass at top showing silvered dial, horizontal quadrant scale.
Non-linear paper-scale allowing readings up to 160 volts. The scale which extends over 70o, is calibrated between 40 and 160 volts, and approximately linear between 60 and 100 volts. Smallest divisions 1 volt. The moving element below has attached an aluminium pointer needle.
Central brass suspension tube extends vertically from the glass. The tube is a modern replacement.
A zeroing screw is found in the base, and two terminals, on the side of the brass case, one of which is labelled 'CASE' and a shorting switch for discharge of quadrants.
A specially-shaped dumbell needle is rotated by electrostatic attraction to charged conducting quadrants, between which it is suspended. Opposite quadrants are connected to earth (one set), and the potential of the needle (other set), which is the voltage being measured. This causes a deflection of the needle proportional to the square of the difference in voltages of the quadrants - hence the non-linear scale.
As the deflection is an even function of the voltage difference, i.e. always in one direction, the voltmeter can be used for AC measurements. Since no current flows through the device (the two terminals are isolated), the voltmeter is almost ideal.
Practically all electrostatic voltmeters are founded on the types developed by Lord Kelvin. The voltmeters were first produced in 1888, and later models were able to measure voltages up to about 20,000 volts. This type of voltmeter was in use for laboratory of industrial measurements as late as 1922.
References:

  1. K. Lyall, The Whipple Museum of the History of Science Catalogue 8. Electrical and Magnetic Instruments numbers 210 and 213.
  2. J. Duncan and S.G. Starling, A Text Book of Physics (1944), Macmillan & Co.
  3. Glazebrook, R., ed., A Dictionary of Applied Physics, Vol. II Electricity (1922), p9-10.

P.D.

041 LINDEMANN ELECTROMETER
Cambridge Instrument Co.Ltd.,/C451401
Original made approx 1932 (C169220), Spare (currently mounted) made approx 1947.
Electrometer dimensions: 36(h)x56(dia)mm. With mounting box: 5(h)x8(w)x10(1)cm.
With microscope 27*8*10
Cylindrical brass case with four brass terminals, windows on top and bottom (removable for 'spare') showing suspended needle inside, surrounded by four cross-connected plates. Mounted on a black box, with hole covered in frosted glass in wall, leading to a prism directing light through the bottom window. A swivelling microscope with scale (E. Leybold's/Nachfolger A.G./Coln - Rhein) also mounted for viewing the needle.
Designed by Prof. F.A. Lindemann for use in connection with photoelectric measurements of light in astronomical work (and also used to measure radioactive emission in an ionisation chamber). Operation is similar to that of a quadrant electrometer. It has a high sensitivity and a stable zero, and does not require levelling. The needle is attached to a torsion fibre, which is fixed under tension to fix the centre of rotation. The voltage across the plates (which replace the quadrants) induces a force on the needle, which in equilibrium is balanced by a restoring force from the fibre. Since no mirror is needed, moving parts only have a small inertia, therefore keeping the period low. An earth terminal eliminates problems with stray capacitance.
References:

  1. K. Lyall, The Whipple Museum of the History of Science Catalogue 8, Electrical and Magnetic Instruments, items 205-208.
  2. Cambridge Electrical Instruments for D.C. Measurements, p70.
  3. F.A. & A.F. Lindemann and T.C. Kelley, "A New Form of Electrometer", Phil. Mag. S. 6. Vol. 47, No. 279, March 1924.

M.L.

028 VAN DE GRAAF ACCELERATOR (1.2 MeV)

High Voltage Engineering Corp. Burlington, Massachusetts, USA. Model #AN2000

230*66D

Cylindrical internal unit only. Pressure chamber missing. Metal shell terminal mounted above apparatus allowing vision of internal structure. Complete with charge carrying belt driven by motor at base and gauges for measurement of ion source gas. 66 cm dia concentric cooling coils surround the base.

A rubber belt carries charge up to the top where it collects on the polished dome raising the potential by up to 1.2 million volts.

Once purchased second hand from Australian National University, the accelerator was used by the Physics Department's beam foil group, at The University of Queensland. In the paper referred to below, the Van De Graaf generated a collimated beam of 40Ar+ ions at 800 keV which interacted with a carbon foil. The polarisation of excited levels in these ions was measured and the effect of polarisation resulting from optical cascades on time-resolved quantum beat measurements was investigated.

References: P.G. Christiansen, et al., J. Phys. B: Atom. Molec. Phys. Vol. 10, No. 17, 3559, (1977).

JD

320 VOLTMETER, KELVIN VERTICAL ELECTROSTATIC (c 1920)

Kelvin, Bottomley and Baird, Glasgow and London, No. 21109

48*37*22

Housed in rectangular brass case painted black, with glass front, rigidly fixed to wooden base with 3 levelling screws. Aluminium pointer includes 3 weights - 28, 84, 333 mg. Comes with mahogany carry case with brass fittings. "Manufacturer's instructions for use of vertical electrostatic voltmeter" pinned to inside of door. 3 ranges: 2500V, 6000V, 12000V.

Multicellular type electrostatic voltmeter, i.e. movement is in the plane of the attracted conductor. Probably used to measure voltages for trains/trams. Has the useful property that large surges of power through it will not harm it unlike a galvanometer based voltmeter.

References

1. K. Lyall, Whipple Museum of the History of Science Catalog 8, 1991.

2. R. Glazebrook, Dictionary of Applied Physics, Vol. III, MacMillan & Co., London 1922.

TG

265 CARBON FILAMENT LAMP

Edison and Swan United Electric Company

Circa 1883-1904

 

-220-16-A1 /[coat of arms, lion and unicorn] BY APPOINTMENT / ROYAL EDISWAN / 8988/10

 

9.5 cm height, 5.2 cm globe diameter. Coiled carbon filament (intact) housed in clear glass globe. 'Bayonet' cap consists of cylindrical brass collar and ceramic base through which platinum wires, insulated from brass and each other by vitrite, connect two ends of filament to power supply.

 

Filament incandesces when current passes through it providing light. Bulb either evacuated or filled with inert gas encapsulates filament to prevent it oxidising. Markings - 220-16-A1 suggest lamp operates at 220V, candle power 16 and draws current 1 Amp. Carbon filament lamps were first practically produced independently by T.A. Edison in the USA and J.W. Swan in England between 1878 and 1880. They provided the solution to the "division of light" problem - that of having small amounts of lighting in different locations without the need to co-locate a power source with each lamp and without excessive transmission line losses. The high resistance of the filaments (i.e. 100 ohms plus) allowed energy to be distributed at high voltages with only low current required to provide sufficient power and thus minimal power loss through copper wires connecting lamp and source. These lamps dominated the market for some 20 years until 1904 when the General Electric Company of America produced the metallised carbon filament lamp, and the highly refractory properties of tungsten began to be realised. The Edison and Swan United Electric Company Ltd arose in 1883 when a lawsuit regarding British patents concerning carbon filament lamps resulted in a merger between Edison's and Swan's English companies.

 

References:

1. R. Glazebrook (Ed), Dictionary of Applied Physics, Vol. II (1922), Macmilland & Co., Ltd., London, pg379-381.

2. H.T. Davidge, R.W. Hutchinson, Technical Electricity, Fourth Impression (Third Edition) (1912), University Tutorial Press Ltd., London, pg276-280.

W.N

345 RESISTANCE BOX (WHEATSTONE BRIDGE)

Otto Wolff, Berlin 5455

37*24*21 cm

The top is in thick ebonite and the brass blocks are lozenge-shaped and undercut. Provided with resistance coils of thick wire for preventing heating when using strong currents. Twin semi-cylindrical contact surfaces are connected by insertion of a plug; this shorts the resistance coil on the bobbin. The 37 plugs are interchangeable and have ebonite handles.

A resistance box is a two-terminal variable resistance so designed that any desired known non-inductive resistance may be introduced into the circuit of which it is a part. In the dial-type box several dials are arranged in decades, so that internal switches select the desired resistance. For example, one dial may select any number of thousands of ohms from one to nine. The next, any number of hundreds of ohms from one to nine; the next, any number of tens of ohms, and the fourth, any number of ohms from one to nine. Thus by manipulating all four dials, any value from 1 to 9999 ohms may be set to the nearest ohm. In the plug-type box, such as this made by Otto Wolff, resistance is inserted by removing a plug, which, when in place, short-circuits the resistance. The plugs must, therefore, be kept clean, and when removed from the box, placed on a clean sheet of paper. They are inserted with a clockwise motion, with only slight pressure. When a slight resistance to turning is felt, a good contact has been obtained.

In fact this device is configured as a Wheatstone bridge, with terminals and key switches for a battery (B) and galvanometer (G). An unknown resistance can be clamped between the heavy clamp connectors provided with silver contacts and the bridge balanced with an equal resistance of unshorted coils between 0.1 and 50 000 W.

The coils are of manganin resistance wire, the resistivity of which does not change appreciably with small changes of temperature. The accuracy of such a resistance standard may be taken as ± 0.25%.

References:

1. Physical Laboratory Apparatus Catalogue 592, Griffin & George (Sales) Limited.

2. Illustrated and Descriptive Catalogue of Physical Apparatus - Including Apparatus for Teaching, H.B. Selby & Co. Pty. Ltd.,

3. Physics Laboratory Manual, Wall & Levine, Second edition, Prentice-Hall 1962.

4. Advanced Physical Physics for Students, B.L. Worsnop and Flint, Ninth edition, Methuen & Co. Ltd.

KW

350 LECLANCHE CELLS

Late 19th Century

Two Glass square-section vessels with a circular openings on top. Base of vessels 9.5*9.5, height 16, total height 20. F2/4 embossed in base of one. Ceramic pots 7cm diameters.

Invented by French scientist Georges Leclanche (1839-1882) in 1866, this battery in its usual form uses a central carbon electrode surrounded by manganese dioxide in a porous (old form) or agglomerate carbon blocks (new form), with zinc rod for the positive terminal placed in a solution of ammonium chloride (sal ammoniac) in a glass bottle outside the central electrode. To maintain the efficiency of the cell it should be placed in a cool dry situation, a little water added occasionally to compensate for the evaporation of the liquid and at intervals, a little sal ammoniac. With a little rest, any accumulation of hydrogen becomes oxidised and the cell recovers its power.

The e.m.f. of a Leclanche cell is about 1.5 volts buts its resistance may amount to several ohms when a porous pot is employed. It was used extensively for telegraphy, signalling and electric bell work; and for most work where intermittent current is required and where it is essential that the battery should require very little attention from time to time.

References:

1. John Griffin and Sons XIV Ed. Catalogue of Scientific Apparatus.

2. H.T. Davidge and R.W. Hutchinson, Technical Electricity, University Tutorial Press, London, 1912, p230.

3. Charles Mollan and John Upton, The scientific apparatus of Nicholas Callan and other Historic Instruments, St. Patrick's College, Maynooth, 1994.

SV

357 HF Ammeter

A.E.Dean, London, No. 93371

  7*15D

    The ammeter was made by A.E.Dean of London and is estimated to be used in the early part of this century for medical purposes.  The ammeter is enclosed by a 15cm diameter case of brass which is  approximately 7cm in depth  and has a glass face.  The ammeter is connected to a terminal and stand on either side of the glass face. The terminals and stand are mostly brass as well.   There is a 5cm long cylindrical non-conducting piece attached to the base of the ammeter stand, which was used to secure the ammeter to an external device.  Thus it might be that the ammeter was a portable one.
    It has been suggested that the ammeter was used for high frequency currents, due to the H.F. that is imprinted on the face.  A high frequency ammeter would have been used in the early part of this century to measure the current in the medical process, called Diathermy.  Diathermy is a medical operation that uses high frequency currents to heal broken and wounded tissue in the human body.
    The internal structure of this ammeter is typical, the input current flows in through the terminals and is split by a parallel circuit of 3 coils of wires which can control the sensitivity of the ammeter, and a gelatine looking sandwich plate that is attached to the roof of the ammeter.
    The actual function of this plate is unknown but it is expected that this must be some sort of rectifier if this ammeter was to be able to use AC signals, and hence high frequency signals.   This plate attached to the roof and the three coils are also in parallel with a standard galvanometer.
    The galvanometer consists of a curved coil of wire and a needle placed near to the coil.  A current flowing in the coil produces a magnetic field which can move the needle.  The amount the needle moves is dependent upon the strength of the current.
       Conclusions about this ammeter are hard to make since there is nothing known of the origin or the donor. Ammeters of similar appearance have been noticed on x-ray equipment.
 
 

References: Medical and Biological physics by Webster and Robertson.
 

J.W.

359 INDUCTION COIL, FORD IGNITION (c.1920)

(Maker unknown)

13*8.5*5.3

The coil is concealed by a wooden housing. An interrupter mechanism is situated at one end with a disc connecting to either a contact on a brass plate or the core of the coil. The core is made of a bundle of iron wires.

The induction coil is a source of high voltage. When a current is switched on, the primary coil is energised and the core becomes strongly magnetic causing the contacts of the interrupter mechanism to separate. This breaks the primary circuit, therefore stopping the current flow in the primary coil. The collapsing magnetic field in the primary induces a large back emf in the coil and the only path for the back-current is across the air gap between the contacts. A spark is produced.

The primary coil then no longer has current flowing through it and therefore the core is no longer magnetic which allows the contacts to connect again remaking the primary circuit.

The cycle then repeats, and in this way, sparks in the cylinders of the engine of a Ford Model T car were produced.

References: Charles Mollan and John Upton, The Scientific Apparatus of Nicholas Callan and Other Historic Instruments, St. Patrick's College, Maynooth, 1994.

 

361 MOVING IRON AMMETER

The Australian General Electric Company / Sydney & Melbourne / Type G.C / No. 77080 /

21 dia.

Black enamel cast iron wall mount moving iron ammeter, 30 amp full scale.

First quarter twentieth century?

In the cut-away diagram above we can see the two iron pieces, one fixed to the body of the ammeter and the other to the needle spindle. The current that passes through the coil induces a magnetic field inside the coil inducing poles in the iron pieces which causes them to repel each other as the current flow increases. As one iron is fixed and the other is attached to the needle, the needle moves around. The calibration is achieved via the vertical adjustment for the response (or the non-zero calibration). The adjustment that is in line with the needle is for the zero point calibration. The restoring force is gravitational. The air damping is achieved using a tube closed at one end and fixed to the body of the ammeter with a plunger fixed to the needle spindle thus damping any movement of the needle and giving a more controlled and balanced response to current variations.

Reference:

  1. Notes and Sales Letters pertaining to Weston Electrical Measuring Instruments, Weston Electrical Instrument Co, Newark, USA, 1922, p.44.

 

R.T.T.

368 STANDARD VARIABLE INDUCTOR (c.1910)

M.W. Sulllivan Ltd / London / Type AA / No. 42

17*17*24

Apparatus consists of two coils of wire, one fixed, one rotatable, mounted in series with a common vertical axis. The coils are mounted in a mahogany box, with an ebonite half circle gauge, graduated from 0º to 180º to iindicate the position of the inner rotatable coil relative to the fixed external coil. The wire of the coils appears to have been wrapped in silk for insulation.

The range of inductance is approximately 5 to 50 mH. When 0=0º, L=L1+L2-2M5mH, where L1,2 are the individual inductances of the two coils, and M is the mutual inductance of the coils. When 0=90º, L=L1+L2, and when 0=180º, L=L1+L2+2M50mH, so it can be seen that the bottom range of the instrument can never be zero.

The instrument is similar in design to the Ayrton and Perry Variable Standard Inductance, first invented in 1895, although it most likely has less loss at higher frequencies due to the decrease in the number of metal parts.

References:

  1. F.A. Laws, Electrical Measurement. McGraw-Hill Book Co Inc, New York. 1917, p.345.
  2. R. Glazebrook (Ed). A Dictionary of Applied Physics, VII. Macmillan. 1922, p.240.
  3. K. Lyall. Whipple Museum of the History of Science, Catalogue 8. Entry 388

370 CAMPBELL ADJUSTABLE MUTUAL INDUCTANCE

Robert W. Paul, London, N, (c.1910)

This Campbell Adjustable Mutual Inductance is splendidly housed in a solid mahogany box with a one-piece timber lid. The top contains seven brass terminals, two rotary range switches and a semi-circular scale. "Total 10 000 microhenries" is stamped in the middle of the scale. Inside are mounted two fixed primary coils and two fixed and one moving secondary coil.

The Campbell Mutual Inductometer utilises the feature of opening out the scale very much as the readings diminish down to zero. This allows the smaller values to be read with much greater accuracy, thus extending the useful working range. This effect is achieved by mounting the primary and secondary coils such that one is kept fixed and the other can be rotated about an axis with the mean plane of one always parallel to that of the other.

This particular inductometer is a long range type, which incorporates strands of wire on both the primary and secondary coils connected in series to enable inductances to be added or subtracted by connecting particular strands. In this way we can either step up to multiples or down to submultiples of the inductance and obtain an accuracy of subdivision in the lower part within 1 part in 10 000 and in the higher part up to 1 part in 100 000.

The arrangement of coils and connections used is illustrated in Fig 1. This shows two equivalent primary coils P and P1 on fixed bobbins. The secondary circuit consists of movable coil S and fixed coils BB and A, which can be put in series depending on the terminals used. The pointer and scale belong to the movable coil, while the fixed secondary coils each consist of ten equal sections which are selected via the two stud-switch dials. The first reads in steps from 100 up to 1000 microhenries and the second from 1000 up to 10 000 microhenries.

References:

  1. The Whipple Museum of the History of Science Catalogue 8 - Electrical and Magnetic Instruments.
  2. Glazebrook, R. (Ed.), A Dictionary of Applied Physics VII, London. Macmillan, 1922, pp. 415-417.

T.S.

164

436

120

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