Helping San Diego, California and beyond since 1997.
Do you need
volunteer, community service, work, military or court hours?
|
|
Helping San Diego, California and beyond since 1997.
|
|
Click here and add this page to your favorites!
|
Encyclopedia Britannica - Main :: SCY-SHA |
|
|
SEISMOMETER (from Gr. veurp6s, earthquake, and fpov, a measure) . This name was originally given to instruments de-signed to measure the movement
sharp
drawn
Popularly it is supposed that earthquake recorders are instruments so sensitive to slight vibrations that great care is necessary in selecting a site for their installation. Although this sup- position Setsmois correct for a certain class of apparatus, as for Se sm example that which will record rapid elastic vibrations pro- duced by the movement
standing
' Up to the middle of the 15th century "seisin " was applied to chattels equally with freeholds, the word " possessed " being rarely used. In course of time the words acquired their modern meaning. See F. W. Maitland, "Seisin of Chattels," Law Quarterly Review, vol. I. p. 324 and The Mystery of Seisin," Law Q. R. ii. 481. Pollock and Maitland, Hist. Eng. Law, vol. ii. 29 seq.; Fry, L. J., in Cochrane v. Moore (189o), 25 Q.B.D. 57. 590 shown that contrivances of this order are wanting in sensibility, and often remain standing
The next class of instruments to be considered are seismometers or earthquake measurers, and seismographs or instruments which Setimo- give diagrams of earthquake motion. Although a seismometer. graph may be designed that will not only respond to Selsmo- fairly rapid elastic vibrations, but will also record very :mph. slow and slight undulatory movements of the ground, experience has shown that the most satisfactory results are obtained when special
special
Experience, however, has shown that even when the movements of the ground are alarming the actual range of motion is so small that a satisfactory record can be ob- tained only by some mechanical (or optical) method of multiplica- tion. This is usually accomplished as shown in fig. 2. b is the bob of a pendulum, with its style s passing through a slot in the short arm of a light lever, sop, pivoted at o, and with its outer end resting upon a revolving cylinder covered with smoked paper . As shown inthe figure, it is evident that the motion of o in the line sop would not be recorded, and to obtain a complete record of horizontal
movements it is necessary to have two levers at right angles to each other. A complete arrangement of this kind is shown in the plan of fig. 2. Here the style s of the pendulum rests in slots in the short arms of two writing levers pivoted at o and o'. Motion of the around in the direction os actuates only the lever so'p', motion In the direction o's actuates only sop, whilst motion in inter- mediate directions actuates both. The length of the short arms of the levers is usually * or II, of the long arms. This type of apparatus has been replaced in Japan by what are called duplex pendulum seismographs. The change was Duple: made because it frequently happened that in consequence pendu- of the movement of the ground agreeing with the period lams. of the pendulum, the latter no longer acted as a steady point, but was caused to swing, and the record became little better than that given by a seismoscope. Very long pendulums (30 to 40 ft.) are less subject to this disadvantage, but on the other hand their installation is a matter of some difficulty. A duplex pendulum (fig. 3) consists of an ordinary pendulum diagrammatically represented by ab, connected by a universal joint to an inverted pendulum dc. The latter, which is a rod pointed at its lower end and loaded at c, would be unstable it it were not connected with b. Now imagine this system to be suddenly displaced so that a moves to a' and d moves to d'. In the new position b would tend to follow the direction of its point of support, whilst c would tend to fall in the opposite direction, and the bob of one pendulum would exercise a restraint upon the motion of the other. If, as in practice, the moment of b is made slightly greater than that of c, the system will come slowly to a vertical position beneath a'd'. In this way, by coupling together an ordinary pendulum about 3 ft. in length with an inverted pendulum 2 ft. 6 in. long, it is easy to obtain the equivalent of a slowly-moving very long pendulum which is too Ic sluggish to follow the back-and-forth movements of its supports. FIG. 3. To complete an instrument of this description (see fig. 4) a point in the steady mass b is used as the fulcrum for the short arm of a light-writing index. This has a ball joint at s, a universal joint at o and a writing point at p, resting upon a piece of smoked glass. Attention was first directed to the possibility, of rendering ordinary pendulums more truly astatic by Professor homas Gray, who suggested methods by which this might be accomplished. The method shown in fig. 4 is that devised by Professor J. A. Ewing. Records obtained from instruments of this description give information respecting the range and principal direction of motion, and show us that in a given earthquake the ground may move in many azimuths. For obtaining an open diagram of an earthquake the best type of apparatus consists of a pair of horizontal
op, free to swing like a gate round a vertical or nearly lams. vertical axis, oo', and loaded at some point b. In practice the weight b is pivoted on the rod whilst its outer end, bp, which writes on a smoked surface, is made extremely light. When the frame of this arrangement is rapidly displaced through a small horizontal range to the right and left of the direction in which the rod points, the weight b by its inertia tends to remain at rest, and the motion of the frame, which is that of the earth, is magnified in the ration op to bp. This apparatus, of which there are many types, was first introduced into seismometry by Professor Ewing. To obtain a complete record of horizontal motion, two of these pendulums are placed at right angles; and by cranking one of the writing levers, o'p', as shown in the plan of fig. 5, two rectangular components of the earth's movements are written side by side. Since the movements of the ground are frequently accompanied by a slight tilting, which would cause b or b' to swing or wander away from its normal position, a sufficient stability is given to the weights by inclining the axis of the instrument slightly forwards. Although by compounding corresponding portions of the diagrams given by instruments of this type, it is possible to determine the range and direction of the movement of which they are the resolved parts, their chief
S 31. A of a pillar, may be projected, whilst the latter measures the effort which have originated at great distances, but for local disturbances, exerted by an earthquake to overturn or shatter various bodies. If after a heavy earthquake we find bodies that have been projected or overturned, then by observing the distance of projection, and the height through which they have fallen, or their dimensions, we can by means of simple formulae calculate quantities closely agreeing with those obtained from the seismogram. For example, if a body, say a coping-stone, has been thrown horizontally through a distance a, and fallen from a height b, the maximum horizontal velocity with which it was projected equals ,/ (ga2/2b); or if the height of the centre of gravity of a column like a gravestone above the base on which it rests is y, and x is the horizontal distance of this centre from the edge over which it has turned, then the acceleration or suddenness of motion which caused its overthrow is measured, as pointed out by C. D. West, with fair accuracy by gx/y. To measure vertical motion, which with the greater number of earthquakes is not appreciable, a fairly steady mass to which a dray's multiplying light-writing index can be attached is ob- Bray S tained from a weight carried on a lever held by any seism- form of spring in a horizontal position. Such an arrange-grph' ment, for wich seismologists are indebted to Professor T. Gray, is shown in fig. 6, in which B is the mass used as the steady point. This, when supported as shown, can be arranged to have an extremely slow period of vertical motion, and in this respect be equivalent to a weight attached to a very long spring , an alternative which is, however, impracticable. The value of these records, as is the case with other forms of seismographs, is impaired by pronounced tiltings of the ground.We next turn to types of instruments employed to record earthquakes which have radiated from their origins, where they may have been violent, to such distances that their move- ments are no longer perceptible. In these instruments the same principles are followed as in the construction of horizontal pendulums, the chief
quakes. consists of a weight of 300 kilos suspended by a wire 25 metres in length, the movements of which by means of writing indexes are multiplied 12.5 times. With pendulums of shorter length, say 2 metres, it is necessary to have a multiplication 8o to loo fold by a double system of very light levers, in order to render the extremely slight tilting of their support perceptible. This arrangement, as devised by Professor G. Vicentini of Padua, will yield excellent diagrams of the gentle undulations of earthquakeseven if the bob of the pendulum acts as a steady point, the highly multiplied displacements are usually too great to be recorded. In Japan, Germany, Austria, England and Russia horizontal pendulums of the von Rebeur-Paschwitz type are employed, which by means of levelling screws are usually adjusted to have a natural period or double swing of from 15 to 30 seconds. These pendulums are usually small. The swinging arm or boom is from 4 to 8 in. long horizontally, and carries at its extremity a weight of a few ounces. A simple form, which is sometimes referred to as a conical pendulum, may be constructed with a large sewing needle carrying a galvanometer mirror, suspended by means of a silk or quartz fibre as shown in fig. 7. To avoid the possibility of displacements due to magnetic influences, the needle may be replaced by a brass or glass rod. The adjustment of the instrument is effected by means of screws in the bed-plate, by turning which the axis o'o" may be brought into a position nearly vertical. As this position is approached the period of swing becomes greater and greater, and sensibility to slight tilting at right angles to the plane of o'o"m is increased. The movements of the apparatus, which when complete should consist of two similar pendulums in planes at right angles to each other, are recorded by means of a beam of light, which, after reflection from the mirror or mirrors, passes through a cylindrical lens and is focussed upon a moving surface of photographic paper . The more distant this is from the pendulum the greater is the magnification of the angular movements of the mirror. With a period of 18 seconds, and the record-receiving paper at a distance of about 15 ft., a deflection of I millimetre of the light spot may indicate a tilting of h part of a second of arc, or 1 in. in 326 miles. Although this high degree of sensibility, and even a sensibility still higher, may be required in connexion with investigations respecting changes in the vertical, it is not necessary in ordinary seismometry. A very sensitive modified von Rebeur instrument was employed by O. Hecker in his measurement of the variation in the vertical and of tidal earth tremors.A type of instrument which has sufficient sensibility to record the various phases of unfelt earthquake motion, and which, at the suggestion of a committee of the British Association, has been adopted at many observatories throughout the world, is shown in fig. 8. With an adjustment to give a 15-second period, a deflection of i mm. at the outer end of the boom corresponds to a tilting of the bed-plate of o"5, or I in. in 6.4 m. The record is obtained by the light from a small lamp reflected downwards by a mirror so as to pass through a slit in a small plate attached to the outer end of the boom. The short streak of light thus obtained moves with dumanaeOItO,punnIr!I Il ^ NWa '\\\\\\\\\\\\\\~ Boom 592 the movement of the boom over a second slit perpendicular to the first and made in the lid of a box containing clockwork driving a band of bromide paper. With this arrangement of crossed slits a spot of light impinges on the photographic surface and, when the boom is steady, gives a sharp
An apparatus of great value in measuring slight changes in the vertical which have a bearing upon seismometrical observation is the Darwin bifilar pendulum. This consists of a mirror about half an inch in diameter, which, when it is suspended as shown in fig. 9, rotates by tilting at right angles to the paper. By this rotation a beam of light re- flected from the surface suffers displacement. It is possible to adjust the apparatus so that a tilt of Tina sec. of arc, or a change of slope of 1 in. in woo miles, can be detected. (See Sir G. H. Darwin, Scientific Papers, vol. i. (1907)) The principle of the Vicentini instrument described above has been adopted by G. Agamennone, director of the observatory at Rocca di Papa, near Rome, and also by E. Wiechert of Gottingen. In the Agamennone seismometrograph the pendulum is cheese-shaped, and weighs 500 kilos in one form and 2000 kilos, or over two tons, in the largest. This cylinder, which is suspended from a stand rigidly attached to the earth, has a vertical hole in its :entre extending from its upper surface to its centre of gravity, and to the bottom of this well a light rod is fixed. The motion of the frame is communicated to this rod by an extension of the frame which makes contact with it just above its point of attachment to the well. The motion is first magnified by the lever, and, on its communication to a complex lever system above the stationary mass, is still further magnified before registration, which is effected by a pen supplied with ink writing on white paper. Mechanism is provided whereby the speed of the paper is doubled on receipt of a shock, an electric bell ringing at the same time to summon an attendant. In the Wiechert astatic pendulum seismometer the stationary mass is also cheese-shaped, but it is supported by a conical extension from its base, which balances it on the floor of its case. There is also an extension from the upper surface of the pendulum, in contact with a system of levers and rods attached to the case; an air-dampi:ig cylinder is fitted to annul the free vibrations of the pendulum. The motion of the rod consequent to a motion of the case is modified by the projecting axle of the stationary mass, and after much magnifcation is recorded on a sheet of smoked paper. This instrument was made with a pendulum weight of t too kilos or over a ton; and with a modified construction the weight was increased to 17,000 kilos or nearly 19 tons, portability being obtained by replacing the solid pendulum of the smaller instrument by a shell which can be filled with barytes, a heavy mineral
The Strassburg or Bosch seismograph differs from those just de-scribed in resembling the Milne instrument, i.e. it is a horizontal and not a vertical pendulum. The steady mass, however, is much larger, being too kilos (or 220 lb) ; the magnification is from 8o to too; and the registration is effected on a roll of smoked paper. An air-damping apparatus is attached in order to annul the natural oscillations of the pendulum. Two of these instruments are set up, one in the N.-S. direction and the other in the E.-W. so as to record the two horizontal components. A more popular Strassburg instrument has a stationary mass of 25 kilos. The Galitzin seismograph, devised by Prince Galitzin, is of the same type, but it essentially differs from the Milne instrument in having its pendulum dead-beat; this is brought about by an electromagnetic device. Magnification and registration of the motion is effected in the following way. Attached to the pendulum is a coil of fine wire which moves in the field of a pair of magnets. The currents induced in the coil are led to a dead-beat D'Arsonval galvanometer having the same natural period of vibration as the pendulum. It is found that the motion of the galvanometer mirror faithfully records, except in a few special cases, the motion of the pendulum; the actual record is made on sensitized paper. Two instruments are set up, and the two components are recorded on one strip. End of Article: SEISMOMETER (from Gr. veurp6s, earthquake, and fpov, a measure) If you wish, you can link directly to this article.
<a href="http://jcsm.org/StudyCenter/Encyclopedia/SCY_SHA/SEISMOMETER_from_Gr_veurp6s_ea.html"> SEISMOMETER (from Gr. veurp6s, earthquake, and ... </a> |
|
|
(Previous) SEISIN (from M. Eng. saysen, seysen, in the leg... |
(Next) SEISTAN, or SISTAN (SEJISTAN) |
|
Sponsored Advertisements