Popular Science Monthly/Volume 12/February 1878/The Magnetic Observatory at Madison, Wisconsin
|THE MAGNETIC OBSERVATORY AT MADISON, WISCONSIN.|
A VISITOR to the grounds of the State University at Madison, Wisconsin, might perhaps wonder what could be the use of three small chimneys to be seen standing out of the south side of the university hill. On being told that under the ground is one of the two magnetic observatories of this country, he may be curious to see more. If so, he will go on down some distance past the chimneys, and, turning into a moderately deep cut, will enter the observatory through a tunnel in the hill-side.
Having divested himself of whatever iron he may have about him—his keys, his knife, and even his watch—two doors are successively thrown open, and he is ushered into a low, vaulted chamber measuring seventeen feet square. The darkness, which would be absolute but for the faint gleams of light escaping through the close coverings of three lamps; the silence, broken only by the ticking of two clocks, or the tread upon the paved floor; the strange character of the instruments, which he begins dimly to see—all unite to create a feeling of oppression, as though one breathed the air of some sorcerer's den.
Though the visitor may be interested in what there is to be seen, yet, in the short time he is allowed to stay, he can get but an imperfect idea of it all. He will learn, it may be, the names of the several instruments, and gain a slight knowledge of the manner in which the observations are recorded. He will be told that the observatory was established at Madison in the autumn of 1876 by the United States Coast Survey; and also, perhaps, that the instruments employed were in use at Key West, Florida, and then at Washington, D. C, being moved from the latter place in order not to be so nearly upon the meridian of a magnetic observatory at Quebec, Canada. If desired, the person in charge will show him some of the traces, one of these being nothing more than the crooked path which a moving spot of light, reflected from a mirror attached to either of the magnets, has left upon sensitive paper.
In order to really profit by his visit, he should have gained, beforehand, some idea of what is to be observed and the manner of observing. This information I shall try to give the readers of this article; and then proceed with a description of the observatory and its equipments.
The force called terrestrial magnetism is subject to variations both in intensity and in direction. There are three ways in which a varying force of this kind may be measured:
1. It may be resolved into components acting along three axes, and the intensity of these components measured. I am not aware that this method has ever been applied to the measurement of terrestrial magnetism; probably, because one of the components would be so small as not without great difficulty to be directly measured.
2. We may measure its intensity along some fixed axis, and its angular variation of direction from that axis, in each of two planes intersecting the same. This method is frequently employed. The fixed axis taken is the intersection of the plane of the magnetic meridian and the plane of the horizon, and the angular variations from the axis are measured in these planes, the variation in the horizontal plane being called the "declination," and that in the plane of the magnetic meridian the "dip."
3. We may measure the intensity of its components along two axes and its angular variation in direction from the plane of those two axes. This last is the method in use at the observatory. The axes assumed are horizontal and vertical, and their plane is that of the magnetic meridian. Angular variations from this plane may be measured in any plane at right angles to it, as the plane of the horizon, and are, therefore, changes of declination.
The instruments used for making the measurements are the declinometer, the bi-filar magnetometer, and the balance magnetometer.
The declinometer consists, essentially, of a bar magnet so suspended as to turn freely in the horizontal plane. Changes in the position assumed by the bar show changes in declination.
The magnet of the bi-filar magnetometer likewise turns in the vertical plane; but, while the magnet of the declinometer is free to assume any position in that plane, the magnet of this instrument is pulled by a constant force into a position at right angles to the magnetic meridian.
The magnet of the balance-magnetometer, like that of the last two instruments, is in a position at right angles to the magnetic meridian; but, unlike either of the other two, it turns in the vertical plane.
The only effect of the horizontal force is to press the magnet against its bearings, and were the magnet suspended at its centre of gravity, the north-seeking pole would point directly downward in obedience to the vertical force. In reality, the magnet is so suspended as to assume a position approximately horizontal. The force of gravity remaining constant, the magnet will not change its position, except with a variation in the intensity of the vertical component of terrestrial magnetism.
In making the observations, care must he taken that the temperature be kept as nearly constant as possible, a magnet losing about one ten-thousandth part of its power for each degree Fahrenheit of increase in temperature; also, that no magnetic bodies are present to influence the magnets; that the instruments be secured from all mechanical interference; and that all the adjustments be true.
Let us see how these requirements are met. The observatory has been built underground, and has double walls and roof, there being a space of two feet between the outside and inside walls. A differential thermometer placed within shows a daily variation in temperature of but 1° or 2° Fahr. In case of artificial heat being required, a brick stove has been built, with which it would be impossible to cause a sudden change in temperature.
The observatory has been placed without the influence of iron water or gas pipes, and in its construction and furnishing no iron has been employed, all metallic supports, mountings, etc., being of brass, copper, or zinc. The reservoirs of the lamps used have to be taken outside to be filled, since to bring a so-called tin oil-can within the observatory would seriously disturb the instrument. As before mentioned, visitors to the observatory leave outside whatever of iron they may have about them. An abnormal variation in the movements of the magnets at the Key West Observatory is thought to have been caused by the landing of some heavy guns in the vicinity, and their subsequent transportation past the observatory.
The mounting of the instruments upon heavy blocks of stone and their close incasement reduce the chance of mechanical interference to a minimum.
To still further guard against errors of observation, there are special adjustments in the several instruments.
The magnet of the declinometer is suspended by a skein of one hundred fibres of silk, the utmost pains being taken to reduce the torsion to a minimum. The length of skein is at least three feet, so that any residual torsion has the less effect. It seems impossible, however, to entirely get rid of this disturbing element. The records of the Key West observations show that the torsion of the suspension skein changed rapidly during the first five months after the suspension of the magnet, and did not become constant even after six years.
As variations in temperature do not affect the direction of the line of action of the magnetic force, no temperature adjustment is required for the declinometer.
The magnet of the bi-filar magnetometer is rigidly connected with a small glass rod of the same length as the magnet. Over the ends of this rod slip two zinc tubes, of such length as to reach within about five millimetres of its centre. At the inner end of each tube is attached one end of a suspension skein, that passes over a glass pulley three feet above the magnet. The diameter of the pulley is fourteen millimetres; it should, however, equal the distance between the ends of the suspension skein—ten millimetres. The present pulley is to be changed for one of that diameter.
The pulley is turned into such a position that the pull of the suspension skein brings the magnet approximately at right angles to the magnetic meridian. The magnet is in equilibrium under the action of three forces: gravity, the pull of the threads, and the horizontal component of the earth's magnetism. The first two forces being constant, the equilibrium is not destroyed save by a variation in the intensity of the third force. The changes in the direction of that force are never sufficiently great to appreciably alter the position of this magnet. The diameter of the pulley being greater than it should be, increases the leverage of the pull of the threads, and so lessens the ratio of the variation of the horizontal force to the sum of the opposing forces. The delicacy of the instrument is thus slightly impaired.
On each side of the point of suspension of the magnet is a place for a small weight. By weighting the magnet its angular position is slightly changed. A comparison of the effect thus produced with the changes due to variations in the horizontal force gives us a measure of that force. It is, in truth, weighing the magnetism.
Observe what takes place when the instrument is heated. Neither the glass pulley nor the glass rod would be sensibly affected. The magnet, however, would lose some of its power, and consequently be less strongly pulled by the horizontal force, which we wish to measure. To counterbalance this loss of magnetic power, the effect of one of the opposing forces must be diminished by an equal amount. This is effected by the zinc tubes, whose expansion brings the ends of the suspension skein nearer together, and thus lessens the pull of that skein.
I now come to the most delicate of all the instruments—the balance magnetometer. Attached rigidly to the axis of this instrument, and at right angles to the same, is an axis, resting, through the interposition of agate knife-edges, upon an agate plate. By changing the position of small brass balls that screw upon vertical and horizontal arms of this axis, the centre of gravity of the instrument may be accurately adjusted to any desired position. None of these balls weigh over fifty grains, and the distance between two successive threads of the screw upon which they work is only the hundredth part of an inch; yet, if one of those for shifting the centre of gravity horizontally be turned through so much as the twentieth part of a revolution, thus advancing it the two-thousandth of an inch, the instrument will be so tilted as never to right itself. This extreme delicacy is attained by bringing the centre of gravity of the instrument close up under the axis of suspension. To prevent unnecessary wear of the agate knife-edges, there is an arrangement for lifting the instrument off of its bearings, when not in use.
The balance-magnetometer requires a delicate temperature adjustment. For this purpose there is attached to the side of the magnet a small tube containing mercury. Such is the position of the tube that the shifting of the centre of gravity of the magnetometer, due to the expansion or contraction of the mercury, shall just balance the tendency of the north-seeking pole of the magnet to rise or fall with the temperature. Adjusting the tube to its proper position occupied Mr. Suess for five days.
The variations of these several instruments are recorded by photography, each instrument, with its recording apparatus, constituting a magnetograph. A cylinder, turned by clock-work, carries the sensitive paper upon which the record is to be made. A single cylinder, with its sensitive paper, suffices for both the declinometer and the bifilar magnetometer, the cylinder turning between the two instruments and receiving the two records at its opposite ends. A second and vertical cylinder is required for the balance magnetometer. The record of all the instruments is made in the same way. The light from a German student-lamp, after passing through a narrow slit, is received upon a concave mirror carried by the magnet. The mirror throws a thread-like image of the slit upon two cylindrical lenses fixed in the case of the recording instrument. By these lenses the line of light is shortened to a dot, to be received by the sensitive paper.
Were the spot of light stationary, a straight line would be traced upon the sensitive paper, since, by the revolution of the cylinder, the paper would be carried directly forward from in under the light. But, by the movement of the magnet, the image of the slit is made to travel back and forth along the lenses and a more or less eccentric trace left upon the sensitive paper.
In order that the trace may not go beyond the limits of the paper, the magnet must be kept from swinging through more than a small arc. This is effected, in the bi-filar magnetometer, by the pull of the suspension skein acting against the magnetic force. In fact, owing to the too great size of the glass pulley, the magnet does not swing quite freely enough.
In the other two instruments a special arrangement is adopted. Surrounding the magnet and forming a closed circuit, is a rectangle of four flat copper bars. Any movement of the magnet gives rise to a current in the circuit, which tends to pull the magnet back again. Thus, if the north-seeking pole of the magnet in the declinometer be deflected toward the east, a current will be generated, running from south to north along the upper bar of the rectangle, and back along the lower bar. The current, in turn, acts upon the magnet, checking it in its swing toward the east, so that the paper can receive the entire trace. In addition to the concave mirror, each magnet carries a plane mirror to receive the reflection of a scale attached to the back of a small telescope. The telescopes are mounted upon the stands of the recording instruments, and for convenience of observation are provided with diagonal eye-pieces. On looking into either telescope one sees its scale reflected in the plane mirror carried by one of the magnets. When the mirror turns with the swing of the magnet, the scale appears to the observer to traverse the mirror's face. At the beginning and again at the end of each trace a record is made of the division of the scale then covered by the cross-wires of the telescope. Knowing the times at which each trace was started and stopped, and the readings of the scale at those times, it is easy to divide off the paper into spaces corresponding to the hours of the day and into other spaces at right angles to these corresponding to divisions of the scale. An exact record is thus made of all magnetic variations.
Particular interest attaches to magnetic observations on account of the way in which the magnetic state of the earth seems to be influenced by the position of the sun, and to a slight degree by the position of the moon; also from the connection between auroral displays and magnetic variations, curves representing the frequency of either agreeing quite closely with curves representing the area of the sun covered by spots. It is well to remark that the curves representing magnetic variations and auroras lag about six months behind those representing the sun-spot variations. The sun-spot area seems in some way to depend upon the position of the planets. Not only is the earth's magnetism thus, seemingly at least, influenced by the sun-spots, but also some of the phenomena of the weather. These last are, of course, in general, masked by local disturbances; but, lately, a very remarkable agreement has been shown to exist between certain magnetic and barometric traces. Investigations into the causes of magnetic variations and the laws under which these variations occur are made by officers of the United States Coast Survey; and to the head-quarters of this survey, at Washington, are forwarded, each month, the traces obtained at Madison.