and the attached mirror is visible through the
little window. The magnet and mirror are sus-
pended upon the long silk or quartz fibre in the
tube at the top of the instrument. Fig. 1 shows
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Fig. 1. QArsS-WEBEH magxetometeb.
a more complicated form of magnetometer with an attached bar for supporting the magnet whose strength is to be measured, and which is placed at a definite distance from the magnetometer,
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FiQ. 2. KOffLRArwH TYPE OF PORTABLE BIPfLAB MAG.NETOMETEB.
and with its axis pointing toward the magnetom- eter, or at right angles to that direction. A magnetometer set up with its magnet in the magnetic meridian will show the declination (q.v.) and its variations; that is to say, it ■will sliow the deviation between the magnetic north as shown by the needle, and the true north as determined by astronomical observations. When the magnet is larger and supported, for example, on two fibres, as in Fig. 2, then the instrument is so set up that the bifilar suspension holds the magnet at right angles to the magnetic meridian. Fig. 3. hecordixg magsetomfter. The light from the lamp L. shining through the small opening O upon the lens C, is brought to an image of O upon the sensitive paper on the drum D, after being re- flected from the magnetometer mirror M. .-Vny angular motion of M around a vertical axis causes the image to move with reference to A and B. This motion, combined with that of the drum D as it ia revolved b.v clock worli, gives a broken line as the record of the magnetic fluctua- tions. In this position the magnetometer measures the strength of the horizontal force of the earth's magnetic field, because if this latter increases then the bifilar system is twisted a little more, and if the earth's field decreases then the bifilar untwists a little.
A magnet so mounted that it can swing in a vertical plane will similarly give the inclination
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Fig. 4. theodolite fob mag.vetic obsebtations.
of the earth's field, or the 'dip' and its varia- tions. Spots of light reflected from mirrors on the needles of these three last forms and allowed to fall upon a moving strip of photographic paper will give an autographic record of the variations of these three components of the field of the earth's magnetism, namely the declination, inclination, and horizontal intensity. The action of such an instrument is shown by the accom-
