1851 Great Exhibition: Official Catalogue: Class X.: Charles Brooke
144. BROOKE, CHARLES, 29 Keppel Street — Inventor and Designer.
Photographic, self-registering, magnetic, and meteorological apparatus.
The object of this apparatus is to obtain a more perfect knowledge of magnetic and meteorological phenomena by continuous observation of all the changes that occur simultaneously in the various instruments. As the magnetic changes are too minute to actuate continuously any mechanism, however delicate, a record can be obtained by an imponderable agent only, as light.
Even with a staff of assistants so large that the eye of one of them should be constantly applied to every telescope, the results would be liable to errors of observation; besides which, the magnetic changes occasionally occur too rapidly to be continuously recorded by an observer. Since the apparatus has been employed at the Royal Observatory, Greenwich, the number of the staff has been reduced, and the fatiguing process of nocturnal observations in the magnetic department has been entirely superseded. The apparatus consists of-
1. A declinometer.
2. A bifilar magnetometer.
In these instruments, the torsion circle from which the suspension skin hangs is supported by eight brass tubes springing from the four corners of a marble slab (which, when in actual operation, would be cemented on the top of a stone pillar firmly fixed in the ground, and insulated from the floor of the observatory): these tubes, about 4 feet long, converge alternately to four points of the torsion plate; they thus compose a framework possessing great stiffness, To the suspension-frame of each magnet, a plane glass mirror and a concave metallic speculum are attached. The plane mirror is for the purpose of making eye-observations with a telescope in the usual manner. A gas-light or lamp is so placed at a distance of about two feet in front of each speculum, that an image of a small slit in the copper chimney surrounding the burner may fall on the sensitive paper attached to-
3. The registering apparatus. This is placed midway between 1 and 2, and consists of a stand supporting horizontally on friction rollers two consecutive glass cylinders, round the inner of which is wrapped a sheet of prepared photographic paper: the outer or covering cylinder keeps the paper moist during the 24 hours it remains in action. A bent arm, attached to the axis of these cylinders, is carried round by a fork at the end of the hour-hand of a timepiece specially constructed for the purpose. The horizontal motion of the tracing point of light, combined with the vertical motion of the paper, traces out the magnetic curve, which, when the paper is removed from the cylinder, is developed and fixed by the usual photographic processes. A third light is attached to the registering apparatus, for the purpose of drawing a standard or base line on the paper; by the varying distance of any point of the magnetic curve from this line, the magnetic variation is determined. At the distance at which these instruments are placed, an angle of 1° is represented by 2 inches on the paper; but the scale valve may be enlarged at pleasure, by placing them further apart. This instrument is shown in fig. 1.
AA, the declination magnet.
B, a concave speculum attached to the magnet.
C, a plane glass mirror also attached to the magnet, for making observations by a telescope, on the old method, when required.
D, the torsion plate, reading to minutes by two verniers.
E, a frame standing upon the torsion plate. A hook capable of being raised or lowered by a screw, is attached to this frame, from which the magnet is suspended by a skein of untwisted silk fibres.
FFF, a glass box, in which the magnet and its appendages are enclosed, to protect them from the air; for the same purpose, the suspension skein is enclosed in a glass tube G, which passes through a stuffing box H, in the lid of the box.
I, a gas-burner enclosed in a brass chimney, from which no light can escape except a small pencil which passes through a narrow slit K, capable of pencil adjusted by a screw; on the breadth of this slit, the breadth of the register line depends.
LL, a combination of two plano-cylindrical lenses. The pencil of light passing through K, falls on the mirror B, and is reflected to the cylindrical lenses; by these, the image of the slit is condensed to a point of light on the surface of MM, the registering apparatus, consisting of two concentric cylinders, between which the photographic paper is placed.
N, the magnetic curve traced by the point of light.
0, a gas-burner, fixed to the stand on which the cylinders rest.
P, a piano-convex prismatic lens, attached to the top of
QQ, an opaque box, which protects the photographic paper from extraneous light. A pencil of light from 0 passes through P, and is brought to a focus on the surface of the paper.
R, the base line, described by the point of light.
SS, the bifilar, or horizontal force magnetometer.
TT, the apparatus for producing an automatic temperature compensation • this consists of two zinc tubes, which are clamped to a glass rod by two adjustible clamps VV, the suspension skein passes over a pulley X, and the ends are attached to two hooks WW; as the temperature rises, these hooks are approximated to each other by a quantity equal to the difference of the expansion of the glass rod and the zinc tubes, between the clamps VV; and thus the torsion force is diminished; the position of the clamps is so adjusted, that the diminution of the torsion force shall be equivalent to the loss of power in the magnet: and vice versa, when the temperature falls. The magnet, its appendages, and the suspension skein are enclosed similarly to the declination magnet; the glass box, etc., is omitted to avoid confusion. The registration of its movements is likewise similarly effected on the opposite side of the cylinders.
4. A blackened zinc case, which is placed over the cylinders, when in actual operation, to prevent any light from falling on the paper, except the two pencils which describe the magnetic curves, and another which passes through a prism on the top of the case, and draws the base line. N.B.—This prism is placed on the top of the glass case, to show its proper position.
5. A case of the same material, which covers the whole of the apparatus, to protect the sensitive paper from any stray light, as well as to defend the whole from dust, etc.
6. A balanced magnetometer, supported by agate knife- edges, resting on agate planes. The variations of this instrument are similarly recorded on-
7. A registering apparatus, similar to the preceding No. 3, except that the axis of the revolving cylinder is vertical. The top of the inner cylinder rests on a turntable, which is carried round by the hour-hand of a time-piece. In this and in the preceding apparatus, the lines of light reflected from the specula are each reduced to a point, by passing through two cylindrical plane convex lenses placed near the sides of the cylinders.
8. A self-registering barometer. The short arm of a lever carries a float which rests on the surface of the mercury in the lower end of a syphon barometer tube. The long arm carries a light screen with a small aperture in it, which is interposed between the revolving cylinder of No. 5, and a light. The small pencil of light passing through the screen marks the photographic paper, and thus records the changes in the mercurial column. The same light which registers the barometer, serves also to describe the base line for the magnetic curve, by a pencil conducted from the back of the chimney through a tube with a right-angled prism at each end of it. This instrument is shown in fig. 2.
AA, a self-registering barometer, enclosed in a case, resting on a stand.
BB, the upper and lower ends of a syphon barometer, which are of the same diameter, and of large size.
C, a float resting on the surface of the mercury, which hangs in a notch on the short arm of a lever.
D, the pivot on which the lever turns.
E, the long arm of the lever, which carries at its extremity an opaque screen F, with a small aperture, through which a small pencil of light passes.
G, a plate on which the tube rests, which is raised or lowered by a screw.
H, a stand supporting a gas-burner.
I, the register line, described by this pencil of light. The screen F will evidently rise and fall with the column of mercury, and the indications will be amplified in proportion to the length of leverage.
K, a tube with a plano-convex prismatic lens at each end of it, placed at the back of the burner; through this, a pencil of light is conducted in the direction indicated by the dotted line, and describes the base line L. By this arrangement, two pencils are derived from the same source of light, which fall perpendicularly on two remote points of the paper.
M, the balanced magnetometer.
N, a concave speculum connected with the magnet by a bar, to which are attached agate knife edges; these rest on agate planes attached to the supporting frame.
0, a plane mirror for making observations with a telescope in the usual manner.
P, a gas-burner, from which a pencil of light is reflected from the speculum N, and passing through a combination of two piano-cylindrical lenses in the frame Q, describes the register line R.
S, a frame supporting a turn-table.
T, the cylinder resting on the turn-table.
V, the gas-pipe supplying the burners.
9 and 10. Zinc cases analogous to 4 and 5.
11. A wet and dry bulb thermometer, and apparatus for registering the temperature they indicate. The registering apparatus consists of a pair of vertical concentric cylinders, similar to No. 7, supported on a table. The bulbs of the thermometers are underneath the table, through which the stems pass vertically, and are placed between the opposite sides of the cylinders and two lights. A narrow vertical line of light brought to a focus by a cylindrical lens, falls on the stem of the thermometer, and passing through the empty portion of the bore, affects the paper. The boundary between the darkened and undarkened portion indicates the position of the mercury in the stem of the thermometer. Five wires are placed across the slit in the frame, through which the light falls on the stem. They intercept narrow portions of the light, and thus the scale of the thermometer is continuously impressed on the register, as well as the temperature. This instrument is shown in fig. 3.
1, 2, camphine lamps.
3, 4, cylindrical lenses, by which a bright focal line of light is obtained.
5, the psychiometer, or wet-bulb thermometer.
6, the dry-bulb thermometer.
7, two concentric cylinders, between which the photographic paper is placed.
8, the register, as it appears after the impression is developed.
9, one of the rollers of a turn-table, on which the cylinders rest.
10, the frame which contains the timepiece.
11, a bent pin, or carrier, attached to the axis of the cylinders this is carried round by a fork at the end of the hour-hand of the time-piece.
As this apparatus is necessarily placed in the open air, when in actual operation, it is provided with-
12. An inner cylindrical zinc case, with sliding doors, to protect the sensitive paper from light, when the cylinder is removed from, and brought back to, the photographic room.
13. An outer wind and water-tight zinc case, with water-tight doors, for removing and replacing the cylinders, and for trimming the lamps, if lamps are used.
14. A timepiece, to show the arrangement of the train. In order to avoid the unsteadiness of the hour-hand, which in ordinary movements results from the play of the motion-wheels under the dial, the central axis which carries the hour-hand is in the train, and the axis which carries the minute-hand is placed out of the centre. As the forked or carrying arm is firmly attached to the axis, another moveable hand or pointer is added, which travels with the former, and points to the hour. The compensating-bars of the balance of this piece are composed of brass and palladium, to prevent the rate being influenced by proximity to the magnets. The numbers of the leaves in the pinions are all prime to the numbers of the teeth in the wheels with which they are in gear, to diminish the chance of irregular motion from wear, as the face of the piece must necessarily be exposed.
15. An elastic scale of vulcanized India-rubber, stretched on a brass frame, for readily marking the subdivisions of time on the registers, which differ slightly in length.
16. Specimens of the registers obtained by similar apparatus.
17. A lithographic facsimile of one day's work of all the instruments employed at the Royal Observatory, from the volume of "Greenwich Magnetical and Meteorological Observations for 1847," to which the reader is referred for further details, as well as to a series of papers by the inventor, published in recent volumes of the " Philosophical Transactions." The most recent improvement of this apparatus is an automatic temperature compensation, adapted to the horizontal-force magnetometer, 2; and to the vertical-force magnetometer, 6. In the former instrument, this object is attained by approximating the lower ends of the bifilar suspension, by the excess of the expansion of a zinc tube, over that of a glass rod: m the latter, by the weight of a small quantity of mercury enclosed in a thermometer tube attached to the magnets, passing from one side of the centre to the other.
[The skilful application of photography, by Mr. Brooke, to register natural phenomena, with no more labour than supplying the cylinder punctually with prepared paper, is one of the most useful and beautiful uses to which photography has yet been applied. The paper is prepared so as to render it extremely sensitive to light, being first washed with a solution of isinglass, bromide of potassium, and iodide of potassium, in the proportion of 1, 3, and 2, respectively; and when required for use, it is washed with an aqueous solution of nitrate of silver, which causes the paper to be sufficiently sensitive to the action of light, so that if a beam of light be allowed to fall upon it, an impression is made upon that part where the light falls, which becomes visible on being washed with a solution of gallic acid, with a small admixture of acetic acid. A light is placed near a small aperture, through which rays pass and fall upon a concave mirror carried by a part of the suspension apparatus of the magnet, and this reflection falls upon a plano-cylindrical lens of glass placed at the distance of its focal length from the paper on the cylinder. As the magnet is ever varying and making small excursions on one or other side of its mean position, the point of light traces a corresponding zigzag line on the paper. The thermometer apparatus has no mirror and no reflector, the mercury in the tubes themselves intercepting the pencils of light; and thus this apparatus, throughout the day and night, is constantly recording the slightest change of position of the magnets, and the smallest changes of temperature.
The object of the self-registering magnetometer above described is to determine the direction and intensity of the earth's magnetism. Its direction is generally found by suspending a piece of steel previously magnetized, or in other words, a magnet, by parallel threads of untwisted silk, and the bar settles in that position in which magnetism causes it to rest, and which is called the magnetic meridian. The angle between the astronomical meridian and the magnetic meridian, gives the magnetic declination, which is the subject of research with the declination magnetometer; at present this value in London is about 22i° west of the astronomical meridian.
Having determined the declination, the vertical plane is determined in which the force of magnetism is exerted.
The angle which the magnet makes, when freely suspended on this plane from the horizon, is termed the dip. At present, the dip at London is about 68° 40'. The force of magnetism exerted in this inclined direction can be resolved into two forces, the one acting in a horizontal direction, the other in a vertical direction, so that conjointly they shall produce exactly the same force as the single force. The bifilar, or horizontal force magnetometer, is intended for measuring the variations of the horizontal component of the variations of the force of magnetism. It consists of a magnet suspended by two halves of a skein of untwisted silk, kept at a certain distance apart. If an unmagnetized bar were thus suspended, it would remain at rest only in that position in which the two parts of the suspension skein were without twist, and if it were turned out of this position, it would endeavour to resume its former position, with a force proportionate to its weight, and the angle through which it had been turned. This principle is made the means of measuring the force of magnetism. A freely-suspended magnet always endeavours to rest in the magnetic meridian.
The suspended skeins are twisted so as to place the magnet at right angles to its natural position; and when in this position, the line joining the two lower divisions of the suspension thread, is inclined to the line joining the two upper divisions of the threads. In this position the effort of the suspending skein to be without twist, is almost constant, whilst the changes in the horizontal force of magnetism are continual, and from these changes the variations of magnetic force may be found.
The variations in the vertical component of the magnetic dip are the subjects of investigation with the vertical force magnet, which is a magnet placed nearly at right angles to the magnetic meridian. It is kept horizontal, or nearly so, by weights balanced with extreme accuracy, and made to vibrate like a balance; and from its different inclination, the variation of the vertical force of magnetism is determined.—J. G.]