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.

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

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