Magnetometer

The method described above of taking solar transits assumes a very accurate knowledge of time. Less accuracy suffices if one can measure the altitude of the transiting sun or star. To ac complish this some patterns, e.g., Cooke's India pattern designed by Capt. Denholm Fraser, replace B in fig. I by a transit tele scope with vertical scale, while the department of terrestrial magnetism of the Carnegie institution, Washington, uses a com bined magnetometer and theodolite. The magnet K when unpro tected, as shown in fig. 2, and remote from the thermometer, has a somewhat uncertain temperature, especially in field work. In the India pattern it is enclosed along with a thermometer in a wooden box, which has a stud in its base which fits in one of several holes bored in the upper surface of the deflection bar ; this calls for a large compensating weight. A single telescope, in this pattern, serves both for vibrations and deflections. The scale is in the telescope, the collimator magnet having the scale re placed by a cross. The auxiliary magnet used in the deflection experiment carries parallel to itself a hollow aluminium tube, fitted like the collimator magnet with a lens and a cross. When the auxiliary magnet is level with the deflecting collimator mag net the aluminium tube is at the height suitable for sighting by the telescope. The latest form of German magnetometer with an auxiliary arrangement for determining the "distribution con stants" P and Q, has been described by R. Bock.

Other

less exact measurements of D use may be made of compasses of the ordinary or more compli cated types. Explorers having to travel light sometimes cal culate H from observations of total force F with a dip circle. (See INCLINOMETER.) We have H=F cos I where I is the angle of dip. A portable instrument in which measurements of the de parture in the value of H from the value at a base station was measured through the varying torsion of a quartz suspension was devised in 1915 by G. W. Walker. A number of instruments

suitable like Walker's instrument for the minute survey of small areas have been enumerated recently by L. Palazzo.

Coil can be derived by comparisons with an artificial magnetic field produced by a measured current in a carefully constructed coil. W. Watson in 1902 constructed an experimental instrument of this kind with which he obtained re sults of high accuracy. Of late years coil magnetometers of various kinds have been designed in different countries, e.g., by F. E.

Smith in England, S. J. Barnett in the United States and N. Watanabe in Japan. Observations with these instruments occupy a very short time, and if sufficient pains be taken with the coil measurements and the auxiliary electrical outfit, a higher order of accuracy should be obtainable than with the unifilar. Until, how ever, international comparisons have been made some uncertainty must prevail on this point.

For taking observations at sea the instrument is naturally carried in gimbals. For measurements of D alone compasses of various kinds are available. An instrument consisting funda mentally of a compass but having an auxiliary magnet was de signed by L. A. Bauer to measure H as well as D. The sea-de flector, as it is called, has the auxiliary magnet with its axis at right angles to the magnetic axis of the compass, the centres of the two magnet systems being a fixed distance apart in the same ver tical. Using a Lloyd-Creak dip circle (see INCLINOMETER) H can be calculated, as on land, from an observation of total force.

BIBLIOGRAPHY.—References: Abbreviations T.M., for Journal of Terrestrial Magnetism; P.R.S., for Proceedings of the Royal Society;