1911 Encyclopædia Britannica/Seismometer

SEISMOMETER (from Gr. σεισμός, earthquake, and μέτρον, a measure). This name was originally given to instruments designed to measure the movement of the ground during earthquakes (q.v.). Observations have shown that, in addition to the comparatively great and sudden displacements which occur in earthquakes, the ground is subject to other movements. Some of these, which may be called “earth-tremors,” resemble earthquakes in the rapidity with which they occur, but differ from earthquakes in being imperceptible (owing to the smallness of the motion) until instrumental means are used to detect them. Others, which may be called “earth-tiltings,” show themselves by a slow bending and unbending of the surface, so that a post stuck in the ground, vertical to begin with, does not remain vertical, but inclines now to one side and now to another, the plane of the ground in which it stands shifting relatively to the horizon. No sharp distinction can be drawn between these classes of movements. Earthquakes and earth-tremors grade into one another, and in almost every earthquake there is some tilting of the surface. The term “seismometer” may conveniently be extended (and will here be understood) to cover all instruments which are designed to measure movements of the ground.

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. SeismoscopeAlthough this supposition is correct for a certain class of apparatus, as for example that which will record rapid elastic vibrations produced by the movement of a train a mile distant, it is far from being so for the ordinary apparatus employed by the seismologist. What he usually aims at is either to record the more or less rapid movements of the ground which we can feel, or the slow but large disturbances which do not appeal to our unaided senses. Generally speaking, the instruments used for these purposes are not disturbed by the vibrations resulting from ordinary traffic. In almost every household something may be found which will respond to a gentle shaking of the ground. Sometimes it is a loosely-fitting shutter or window-frame, a hanging drawer-handle, or a lamp-shade which will rattle; the timbers in a roof may creak, or a group of wine-glasses with their rims in contact may chatter. Any of these sounds may call attention to movements which otherwise would pass unnoticed. Specially arranged contrivances which tell us that the ground has been shaken are called seismoscopes or earthquake indicators. A small column, as for example a lead pencil standing on end, or a row of pins propped up against suitable supports, or other bodies which are easily overturned, may be used as seismoscopes. Experience, however, has shown that contrivances of this order are wanting in sensibility, and often remain standing during movements that are distinctly percptible. A more satisfactory arrangement is one where the body to be overturned is placed upon a platform which exaggerates the movements of the ground. For example, the platform h (see fig. 1) may be on the top of a small rod r, fixed at its lower end by plaster of Paris in a watch-glass w, and carrying a disk or sphere of lead at l. When the stand on which w rests is shaken, a multiplied representation of this movement takes place at h, and any small body resting on that point, as for example a small screw s standing on its head, may be caused to topple over. If the loaded rod is elastic its lower end may be fixed in a stand, and the spherically curved base w is no longer required. In this case the motion at h is that of elastic switching. Apparatus of this kind may be employed for several purposes beyond merely indicating that an earthquake has taken place. For example, if the falling body s is attached by a thread to the pendulum, of a timepiece, it may be used to stop it and indicate the approximate time at which the tremor occurred. In its most sensitive form r is a steel wire, the upper end of which passes freely through a small hole in a metal plate. By the movement of the wire or the movement of the plate, especially if the latter projects from the top of a second and similar piece of apparatus, an electrical contact can be established by means of which an electromagnet may ring a bell, stop a clock, or set free machinery connected with a cylinder or other surface upon which an earthquake machine may record the movement of the ground.

Fig. 1.

Fig. I.

Seismometer. Seismograph.

The next class of instruments to be considered are seismometers or earthquake measurers, and seismographs or instruments which give diagrams of earthquake motion. Although a seismograph may be designed that will not only respond to fairly rapid elastic vibrations, but will also record very slow and slight undulatory movements of the ground, experience has shown that the most satisfactory results are obtained when special instruments are employed for special purposes.

First we will consider the types of apparatus which are used to record the rapid back-and-forth movements of earthquakes which can be distinctly felt and at times are even destructive. The essential feature in these seismographs is a fairly heavy mass of metal, so suspended that although its supports are moved, some point in the mass remains practically at rest. For small earthquakes, in which the movement is rapid, the bob of a very long and heavy pendulum will practically comply with these conditions. If a style projecting from this pendulum rests upon say the smoked surface of a glass plate fixed to the ground, the vibratory motion of the ground will be recorded on the glass plate as a set of superimposed vibrations. To obtain an open diagram of these movements the plate must be moved, say by clockwork.

Fig. 2.

Fig. 2

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 obtained only by some mechanical (or optical) method of multiplication. 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 in the 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 ground in the direction os actuates only the lever so′p′, motion in the direction o′s actuates only sop, whilst motion in intermediate directions actuates both. The length of the short arms of the levers is usually 1/6 or 1/12, of the long arms.

Duplex pendulums.

Fig. 3.

Fig. 3.

This type of apparatus has been replaced in Japan by what are called duplex pendulum seismographs. The change was made because it frequently happened that in consequence of the movement of the ground agreeing with the period 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 if 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 sluggish to follow the back-and-forth movements of its supports.

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 Thomas 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.

Horizontal pendulums.

Fig. 4.

Fig. 4.

For obtaining an open diagram of an earthquake the best type of apparatus consists of a pair of horizontal pendulums writing their movements upon a moving surface. A simple form of horizontal pendulum as shown in fig. 5, consists of a rod, op is free to swing like a gate round a vertical or nearly vertical axis, oo′, an 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 value is that they enable us to measure with ease the extent of any vibration, half of which is called its amplitude, and the time taken to make any complete back-and-forth movement, or its period. Now if a be the amplitude expressed in millimetres, and t the period expressed in seconds, then the maximum velocity of an earth particle as it vibrates to and fro equals 2πa/t, whilst the maximum acceleration equals 4π²a/t². The former quantity determines the distance to which a body, as for example the capping

Fig. 5.

Fig. 5.

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 multiplying light-writing index can be attached is obtained from a weight carried on a lever held by any form of spring in a horizontal position. Such an arrangement, for which seismologists are indebted to Gray’s seismograph.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.

Fig. 6.

Fig. 6.

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 movements are no longer perceptible. In these instruments the same principles are followed as in the construction of horizontal pendulums, the chief difference being that the so-called steady mass is arranged to have a much longer period than that required when recording perceptible earthquakes. Instruments largely employed for this purpose in Italy are ordinary pendulum seismographs as in fig. 2. Instruments to record distant earthquakes.One at Catania 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 80 to 100 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 earthquakes which have originated at great distances, but for local disturbances, even 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 1 millimetre of the light spot may indicate a tilting of 1/100 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.

Fig. 7.

Fig. 7.

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

Fig. 8.

Fig. 8.

of 1 mm. at the outer end of the boom corresponds to a tilting of the bed-plate of 0′′·5, or 1 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 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 fine line. The passage of the long hand of a watch across the end of the slit every hour cuts off the light, and gives hour marks enabling the observer to learn the time at which a disturbance has taken place. The chief function of the instrument is to measure slow displacements due to distant earthquakes. For local earthquakes it will move relatively to the pivoted balance weight like an ordinary bracket seismograph, and for very rapid motion it gives seismoscopic indications of slight tremors due to the switching of the outer end of the boom, which is necessarily somewhat flexible. If we wish to obtain mechanical registration from a horizontal pendulum of the above type, we may minimize the effect of the friction of the writing index-say a glass fibre touching the smoked surface of moderately smooth paper-by using a considerable weight and placing it near to the outer end of the boom. In the Isle of Wight there is a pair of pendulums arranged as in fig. 5. The stand is 3 ft. in height. Weights of 10 ℔ each are carried at a distance of 10 in. from the pivots of booms which have a. total length of 34 in. With these, or even with booms half the above length, actuating indices arranged as shown in fig. 2, but multiplying the motion six or seven times, good results may be obtained. At Rocca di Papa near Rome there is a pair of horizontal pendulums with booms 8 ft. 9 in. in length, 17 ft. in vertical height, which carry near their outer ends weights exceeding half a hundredweight. Although such apparatus is far too cumbersome to be used by ordinary observers, it yields valuable results.

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 1/1000 sec. of arc, or a change of slope of 1 in. in 1000 miles, can be detected. (See Sir G. H. Darwin, Scientific Papers, vol. i. (1907).) .

Fig. 9.

Fig. 9.

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 centre 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 s eed of the pa r is doubled on receipt of a shock, an electric bell) ringing at tli; 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-damping 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 b the projecting axle of the stationary mass, and after much magnification is recorded on a sheet of smoked paper. This instrument was made with a pendulum weight of 1100 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 readily obtainable in most places. This instrument, which has a magnification of 2200, detects the slightest tremors, and is consefiiuently most useful in recording earthquakes of distant origin; its hig sensitiveness and complications, however, militate against its common use. Wiechert has also constructed a seismometer on the same principle, but in which the stationary mass is smaller, being adjustable between 80 and 200 kilos (180 and 440 ℔).

The Strassburg or Bosch seismograph differs from those just described in resembling the Milne instrument, i.e. it is a horizontal and not a vertical pendulum. The steady mass, however, is much larger, being 100 kilos (or 220 ℔); the magnification is from 80 to 100; 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.

Authorities.—For older forms see R. Mallet's Report of the British Association (1858). For modern forms see J. Milne, Seismology (London, 1898); Transactions of the Seismological Society of Japan, vols. i.-xvi.; Seismological Journal, vols. i.-v. (Yokohama. 1880–1895); Bollettino della Società Sismologica Italiana, vols. i.-v. (Rome, 1895); J. A. Ewing, Memoir on Earthquake Measurement (Tokyo, 1883): Reports of the British Association (1887-1902); E. von Rebeur-Paschwitz, Das Horizontalpendel (Halle, 1892); A. Sieberg, Handbuch der Erdbebenkunde (Braunschweig, 1904).