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.
