128 DIP CIRCLE
Supplied by Cambridge Scientific Instrument Co., Cambridge.
20*20*20
Circular brass base in the form of an annulus mounted on a horizontal tripod, adjusted screws under each arm, mounted on a square wooden plate 20 cm ´ 20 cm. Annulus has angular gradations down to 0.5° resolution, and is inset with a matching brass disk rotating about a central axis, also mounted on the tripod. The inner disk has attached a vernier scale by a screw clamp to the annulus, with a fine rotation screw, for measurement of angular position.

A horizontal, roughly-rectangular platform mounted on the inner disk with a spirit level below to allow accurate levelling via the tripod screws. A vertical box at the back of the platform (15cm ´ 15cm ´ 4cm deep) with glass windows front and back (the rear smoked). Box contains a magnetised needle mounted on an axle suspended freely on two agate edges on metal posts. A knob on the right-hand side of the box raises one support to center the axle. The back window of the box hinges open horizontally. A vertical annulus mounted on the platform via two cylindrical posts, in a plane parallel to the box face, with 0.5° angular markings, and a rotating centrepiece attached via a vernier scale, containing at opposing ends a small microscope used to view the ends of the needle. A swivelling piece on the same axis, with magnifying glasses at opposing ends, used to view the vernier scale. Separate from the apparatus (all contained in wooden box) are two long permanent magnets used to remagnetise the needle. Used to measure the three-dimensional orientation of the prevailing geomagnetic field, to obtain the heading of the vertical plane containing it, and the inclination in that plane. Returns a horizontal heading relative to an arbitrary reference, and vertical inclination, as angular measures in degrees of arc.Does not measure the actual field strength.

The apparatus in the museum, has the inscribed annotation "Supplied by the Cambridge Scientific Instrument Company". However, there is no serial number (so the time of manufacture can not be ascertained), and it is likely that it was built under licence. Identical designs are present in the Casella catalogue, for example, and refer to the apparatus as a Kew Pattern Dip Circle, indicating the source of the design. It is expected that the needle is frequently remagnetised by hand, to ensure that its field axis lies along the line between the needle points. This is necessary to ensure accurate operation, although the effect of small errors across the plane of the needle is accounted for by the averaging of needle-reversed measurements. Special permanent magnets are provided with the apparatus, for this purpose.
References:

  1. Advanced Practical Physics for Students (Worsnap & Flint) pages 475-477, "The Dip Circle Measurement of the Angle of dip"
  2. Casella Catalogue, page 105, item 4518, "Kew Pattern Dip Circle".

S.G.

200 KEW TYPE MAGNETOMETER (157,158,159)
Cambridge Scientific Instrument Company Ltd/Cambridge, England 178
47*68*22
Central housing with removable wooden sides for suspended magnet and rotatable on tripod base with three levelling screws. Top of base contains scaled horizontal circle, two reading microscopes and spirit level. Telescope arm and deflection scale extending from central housing with focussing telescope, spirit level and mounted filters attached atop extension from central housing. From other end of central housing extends the platform for the mirror mount. Glass suspension tube, containing coupling on end of suspension fibre, screws into the top of central housing.

The base instrument (note attachment 158) alone only allows the strength of the Earth's magnetic field to be known. A magnet (see 157) placed in the coupling at the end of the suspension fibre is allowed to oscillate through some small angle. The angle of deflection is read from the deflection scale through the telescope are. From the period, the product of the Earth's field strength and the moment of the magnet may be found.

This same magnet is then placed parallel to and attached to a ruler which connects to the top of the base through the brackets and clamp pins and runs perpendicular to the central housing and telescope arm. From the deflection of a needle due to the competing magnetic fields the moment of the magnet may be eliminated from the above product and the Earth's field strength determined.
References:

  1. B.L. Worsnop & H.T. Flint , Advanced Practical Physics for Students, (1951), page 470-475

F.G.

173 GRAVITY METER (.1960)
C.H.Frost Gravimetric Surveys, Tulsa, Oklahoma, Meter No. C-1-16/Licensed under Hartley Pat./No.2, 159, 062 and Reissue No. 20, 137
40*33D
Externally: large cream coloured cylinder, top of it contains the eye-piece, measuring screw, adjusting & reading terminals, internal parts are exposed for observation above the cylinder; "zero-length" spring at approximately 45° angle to the vertical attaches to a beam with a small mass at the attaching end & at the other end, a cylinder housing the shock eliminating spring. This instrument is a Lacoste and Romberg gravity meter, used to take relative gravity measurements between stations. The gravity meter balances the force of graviy on a mass in the specific gravitational field against the elastic force of the spring. The zero-length spring postioning causes significant mechanical magnification hence a small change in gravity causes a significant beam displacement, which is measurable. Adjustment of the spring is accomplished using the measuring screw. Without a mass attached, the zero-length spring (obtained by pre-stressing the spring in winding) is a finite length and an initial force is required before the coils begin to separate. The zero-length and the shock eliminating springs completely suspend the gravity response -system, i.e. the beam and mass, protecting the delicate instruments from nearly any shocks except those that damage the housing itself. The cylinder is insulated with a foam-like material and as temperature flucuations can cause changes in the measurements, has a temperature control device. Station gravity is generally repeatable to better than 0.1 mgal. It appears that gravity meters using these basic principles are still in fairly common use today. Lacoste and Romberg have also developed gravity meters for use underwater, shipborne, and in boreholes (much smaller systems).
Reference:

  1. McGraw-Hill Encyclopedia of Science and Technology, 7th ed., vol 8, McGraw-Hill, N.Y.,1992
  2. Numerical Data and Functional Relationships in Science and Technology, Group V. Geophysics & Space Research vol.2, Springer-Verlag, Berlin, 1984.p323.

A.B.

031 EÖTVÖS BALANCE (c.1920)
Askaniawerke AG, Berlin(?)
40*54*24
Consists of a torsion balance with two platinum masses suspended at different heights by a fixed platinum-iridium wire. Protected by a triple walled torsion chest which may be turned around the axis of the entire instrument and adjusted in any azimuth on a horizontally divided circle. Mechanism for measuring is dominantly clockwork, also enclosed within torsion chest. Developed in 1888 by Professor Lorand Eötvös, the Eötvös Balance was used extensively until the improvement in the accuracy of gravity meters in 1936. The deviation of the Earth's surface from a sphere produces components of the gravity force in the horizontal plane. This turns a suspended beam in the direction of the smallest principle curvature of the Earth's surface. The wire will twist and the moment of rotation depends on the magnitude and direction of the horizontal directing force of gravity. The wire is then turned back by an elastic force produced by the torsion of the wire. The two moments of rotation equilibrate the beam and thus (using the angle of torsion and moment of torsion of the wire) the moment of rotation of gravity can be determined. As the lines of force of gravitation curve downwards in the direction of the gradient the forces of gravitation are not parallel at different heights. Thus the forces affecting the weights suspended from the beam incline towards the axis of rotation at different angles. The components perpendicular to the axis adjust the beam parallel and proportional to the gradient of gravity. Thus the magnitude and direction of the gravity field can be measured from the inclination and declination of the beam suspending the two platinum masses. In use the whole device is rotated by clockwork and measurements are recorded on a photographic plate. Several hours are needed to measure the gradient at one point.
References:

  1. LeFehr, T.R., (1980), History of Geophysical Exploration: Gravity Method, Geophysics, v.45, pp1634-9.
  2. Eckhardt, E.A., (1940), A Brief History of the Gravity Method of Prospecting for Oil, Geophysics, v.5 p231-42.
  3. Rybar, S., (1923), The Eötvös Torsion Balance, Economic Geology, v18 pp639-62

B.S.

062 THEODOLITE (c.1890)
Troughton and Simms London
130*150D
Brass 5 inch theodolite with 4 screw base and table stand. Telescope eyepiece carries crosshairs for alignment and three bubble levels allow levelling. Vernier scales are etched on vertical circle and on opposite ends of diameter on base. Standard vernier theodolite which uses cross hairs aligned along telescope optical axis to measure horizontal and vertical angles. Vertical angles from 0-45° and horizontal angles from 0-180° can be measured. Both scales can be clamped and then finely adjusted with tangent screws. An attached microscope allows accurate reading of the scales. The construction also permits a plumb bob to be suspended from the centre of the vertical scale.
References:

  1. W.F. Stanley: Surveying and Levelling Instruments
  2. J. Clendinning Principles and Use of Surveying Instruments (pp 64-7)

L.B.

108 & 109 PORTABLE MERCURY BAROMETER
Stores reference No G6C/89 Mark1
A.L. Franklin Sydney Serial No 108 1944
95*16D
The barometer stands approximately one metre high and has its original lacquered and black finish. It is virtually identical to that made by Short and Mason, London (cat no.109). There is a plate attached to the front marked in inches from 6 to 31. A small metal mark with a stopper screw is attached to the front of the barometer. The bottle and the mercury are no longer attached. The glass tube that contains the mercury sits behind the plate in a metal housing. The tube extends below the surface of the mercury into a cistern of much larger cross section than the tube (it was not possible to make the cistern barometer portable until they could make the mercury tube sufficiently small ), so that the rise and fall of mercury is small but not negligible in comparison to that in the tube. To compensate for the change in the mercury level the scale graduations have been foreshortened.. In meteorology mercury barometers are used to measure the atmospheric pressure.
References:

  1. Mercury barometer and Manometers, U.S. Department of Commerce, National Bureau of Standards

M.W.

071 ASSMANN'S HYGROMETER (c1920?)

Negretti and Zambra, London.

41*9*9

Chromed outer casing enclosing clockwork fan and two thermometers. Thermometer bulbs extend into metal ventilation tubes at the base of the instrument. Air is drawn through the tubes, and up to connected shaft running between the thermometers, by a clockwork fan.

Used in the measurement of humidity, and works on the principle of cooling due to evaporation. The bulb of one thermometer is encased by a wick which extends to the end of the corresponding ventilation tube. The wick is moistened and the fan causes an air current of 2-3 ms-1 past both bulbs. The difference in wet and dry bulb temperatures is utilised in calculations of the vapour pressure of water in the surrounding air.

The instrument is designed to be portable, and comes with mounting bracket and hose clamps for the ventilation tubes.

References: Sir Richard Glazebrook, A Dictionary of Applied Physics Vol. III, MacMillan & Co. Ltd., London, 1923, p425.

SC

321 18" CELESTIAL GLOBE

W & A K Johnston Limited, Edinburgh and London

75*75*75

Stand and meridian ring made of brass, wooden horizon ring and paper scale.

Representation of stars and constellations as they would appear on a sphere (Celestial sphere) of infinite radius with the Earth at the centre. Given a particular time and location on the Earth, can be used to determine what will be visible in the sky. Decorated with drawings of the imaginary objects making up the constellations. The background is blue and the stars are in white with the size indicating magnitude down to 6th.

Construction of the first celestial globe is generally credited to Thales of Miletus, 6th Century B.C. and probably the oldest in existence is the Farnese Globe at the Museo Archeological Nazionale at Naples estimated to be from the 3rd Century B.C.

Reference: R. Lister, Old Maps and Globes, Bell & Hyman, London, 1979, pp.71-73.

ST

324 HORIZONTAL SEISMOMETER
Model 8700C, S.N. 190.
Teledyne Industries, Texas, USA. Geotech Division, Garland
A seismometer is a device that transforms the seismic energy of the earth into an electrical signal. This particular version measures the amount of translational motion the earth has undergone. Generally, for an overall picture of each movement one would use this in conjunction with vertical and deformation measuring seismometers.

The 10kg suspension arm, mounted on the flexure pivots at one end of the instrument, forms the horizontal pendulum. This oscillates around a sub-vertical axis with a natural period adjustable between 10 to 30 seconds. For the calibration required at each particular natural frequency there are calibration coils located with the main coils.

The base of the instrument transmits the earth's motion through to the assembly of magnets. The main coils are attached to the end of the suspension arm and are placed between the poles of a pair of strong cup magnets. Pendulum motion induces an electric current in the coils, proportional to the velocity of this relative motion. This induced current also causes electromagnetic damping in the seismometer.

A lamp, aperture, photoresistors and resistors form the Mass Position Monitor, for an electrical indication of the inertial mass position whenever required. A Remote Centering Accessory is also supplied.

At the far end of the instrument there is a small scale with a pin to show deflections. A small circular level on the top of the instrument is used to ensure the equipment is entirely horizontal. The instrument also contains a heater to stabilise conditions and minimise noise. Additionally there is a cover that, when secured, ensures the seismometer is watertight and durable enough to work in unfavourable conditions.

References:

  1. Academic Press, Encyclopedia of Physical Science & Technology, Academic Press Inc., London, 1987.
  2. Advanced Research Projects Agency, High-Gain, Long-Period Seismograph Station Instrumentation (Vol. 1), Lamont-Doherty Geological Observatory of Columbia University, Columbia, 1971.
  3. McGraw-Hill, Encyclopedia of Science & Technology, McGraw-Hill Inc., USA, 1992.

A.B.

325 PHOTOTUBE AMPLIFIER
Geotech/A Teledyne Company/Garland, Texas/Model 5240B/Serial 1463/Assembly 90-0524-20.
30*51*48
Part of a seismograph station which was assembled by the Lamont-Doherty Geological Observatory of Columbia University in 1972 for use in the Geology Department of The University of Queensland.

The phototube amplifier consists of a Kinemetrics Model LG-1 galvanometer, beam splitter, light source and phototube deck, in a sealed case, 30 x 51 x 48 cm, and weighs about 25 kg.

The phototube amplifier is a galvanometer phototube amplifier designed to amplify very low-level voltages or currents in the long period region of the seismic spectrum. Light from the light source is reflected from the galvanometer mirror and focussed on the beam splitter-lens assembly. A phototube is located at the focal point of each half of the beam splitter lens. These phototubes are connected in series across a regulated voltage. The galvanometer is electromechanically adjusted so that when no current passes through the galvanometer, the light is evenly divided between both sides of the beam splitter, and then the voltage at the junction between the two phototubes is exactly half the applied voltage. When the galvanometer rotates, unequal intensities of light fall on each half of the beam splitter and the phototubes register unequal quantities of light, and the voltage at the junction between the phototubes varies accordingly. This voltage is filtered and passed to the output. A gain of 500 000 at .03 Hz is specified.

References:

  1. Lamont-Doherty Geological Observatory (1971) High-gain, long-period Seismograph Station Instrumentation. Vol. I. ARPA Report order no. 1513.

F.N.

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