063 MAGNETIC DIP NEEDLE
W. WILSON/1 Belmont Street, London NW
The magnetic dip needle consists of a brass cylinder with glass faces, 18 cm in diameter. A magnetised steel needle is mounted free to rotate about a horizontal axis through its centre of gravity. The needle will thus settle with its magnetic axis in the magnetic meridian and inclined to the horizontal at the angle of dip. The inclination is then read off the scale on the face marked with graduations 90o (vertical) through to 0o (horizontal). The other face is hinged and may be opened to reset the needle. There is a screw mounted on the side of the cylinder which raises or lowers an arm which catches and supports the needle and holds it in a fixed position for storage and transport. The cylinder is also able to rotate about a vertical axis through its circular base (diameter 15cm) which is also marked with graduations 0o to 90o in each quadrant. There are two holes on either side of the faces, positioned so that a square is formed. These may have been used for a spring arm or, more likely, for attaching a bubble level (though this does not easily explain the holes' positions). The stand has levelling screws on the supports. A small level was probably enclosed with the dip circle in its wooden case. This instrument is likely to have been of a 'sturdy' design, designed for field work.
Compass and dip needles were used to find magnetite in Sweden in the Middle Ages, making the magnetic method the oldest of all applied geophysical techniques. This method of magnetic prospecting for ores was used extensively for iron mining up until the early decades of the twentieth century. However, the dip circle is not a sensitive enough instrument for most ore prospecting purposes and has been replaced with ground-based versions of magnetometers used in aeromagnetic surveys.
Surveying and Drawing Instruments, Catalogue No. 523, C.F. Casella and Co. Ltd., London, p106.
This reference contains information on a magnetic dip circle of very similar design used in India, and provides an illustration.
John Milsom, Field Geophysics, Open University Press, Milton Keynes, 1989, p47.
Dobrin, Geophysical Prospecting (2nd ed.), McGraw-Hill Book Company, New York, 1960, p278.
Valarie H. Pitt, The Penguin Dictionary of Physics, Penguin Books Australia Ltd., Ringwood, 1977.
097 REGNAULTS HYGROMETER
A brass tube that acts as both the system's exhaust and stand, is screwed into a brass and lead base engraved with "Griffin, London". A brass T tube fits into the top of the stand, into which two glass L tubes fit into either arm. Two identical glass cylinders, each closed at one end by a silver thimble, are attached to the L tubes by cork stoppers, one of which has three holes (broken) the other two. One of the thimbles is filled with ether (an organic compound with a lower evaporation temperature than water) and its connecting glass cylinder is stopped with the three holed cork. A thermometer and a glass inlet tube both fit through the cork and end in the ether. Into the other cylinder, a second thermometer (missing) is also fitted. Finally, an aspirator or suction bulb (missing) is attached to the stand and air is drawn through the system; initially passing through the ether.
Before any air is drawn through the system, it is allowed to reach an equilibrium point, at which the atmosphere within the hygrometer is saturated with ether vapor. A small amount of fresh air is then passed through the ether and into the system, while the same amount of saturated air is expelled. This allows for further evaporation of ether and a consequent cooling of the full thimble. The action of the air bubbles in ether serves a twofold purpose in stirring the ether, facilitating more even heat distribution and conduction into the thimble and expediting evaporation. The temperature of the ether (and hence thimble) at which dew is formed can be measured and compared to the air temperature measured by the other thermometer.
As described above, the apparatus functions as a device which is capable of gradually lowering the temperature of a polished surface whilst measuring its temperature. The point at which dew is formed, the dew point, is easily recognizable on the silver, as it suddenly dulls, and this can be referred to in tables which indicate the humidity corresponding to particular dew points at different temperatures. Actually measurements are made of the first appearance of dew, and then its disappearance as the temperature is allowed to rise again, and the results averaged.
Originally, a telescope was also supplied (now missing), as it is necessary to read the thermometers from a distance of about ten meters. This is due to the fact that the average person emits about 63 grams of water per hour. Considering that only five grams are required to saturate a cubic meter of air at zero degrees Celsius such a precaution seems warranted.
1. Glazebrook, Sir Richard, ed. Dictionary of Applied Physics, Vol. III, MacMillan, London, 1923, p417.
128 DIP CIRCLE
Supplied by Cambridge Scientific Instrument Co., Cambridge.
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.
200 KEW TYPE MAGNETOMETER (157,158,159)
Cambridge Scientific Instrument Company Ltd/Cambridge, England 178
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.
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
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).
031 EÖTVÖS BALANCE (c.1920)
Askaniawerke AG, Berlin(?)
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.
062 THEODOLITE (c.1890)
Troughton and Simms London
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.
108 & 109 PORTABLE MERCURY BAROMETER
Stores reference No G6C/89 Mark1
A.L. Franklin Sydney Serial No 108 1944
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.
071 ASSMANN'S HYGROMETER (c1920?)
Negretti and Zambra, London.
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.
321 18" CELESTIAL GLOBE
W & A K Johnston Limited, Edinburgh and London
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,
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.
325 PHOTOTUBE AMPLIFIER
Geotech/A Teledyne Company/Garland, Texas/Model 5240B/Serial 1463/Assembly 90-0524-20.
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.
349 ALTIMETER (c 1940)
Wallace and Tiernan, Belleville, New Jersey, Model No. FA181/Serial No. GG06822
Housed in a cylindrical metal case painted green, with hinged lid, and shoulder strap affixed on opposite sides of the base. The base contains the altimeter, which is simply an aneroid barometer. This is sealed behind a glass face to prevent moisture entering.
The lid has conversion scales "Air temperature and Relative humidity correction factors for Altitude", behind which clips in a hygrometer which is a Wet and Dry bulb thermometer style hygrometer.
This was a U.S. Army field Altimeter which operated by the atmospheric pressure bending the elastic top of a partially evacuated drum, actuating a pointer. The hygrometer can be removed from the lid and spun in the air so that a measure can be made of air temperature and relativity humidity ( which affects the density of the air). After both barometer and hygrometer measurements are taken, the tables are used to evaluate the approximate altitude.
1. Barometer, Microsoft Encarta '95, Funk & Wagnall's Corporation, 1994.
353 SIGHTING COMPASS
Type 06 no. 18774.D
Diameter of bowl 10 cm length 20 cm
Hollow wooden handle with screw on bottom supporting brass bowl. Glass face. Moveable prism with marker for sighting . Mother of pearl magnetic disc mounted on type of universal joint immersed in discoloured liquid in center of bowl. Disc marked with degrees in mirror writing.
To sight compass hold level, look over glass face to prism, line up arrow on prism with pin that protrudes over disc and read of disc.
A Crown with the letters AM etched into the face probably indicate the compass
was used on a government boat.
Continue the Tour to