It remains to consider the measurement of the declination and the horizontal component, these two elements being generally measured with the same instrument, which is called a unifilar magnetometer.
- Unifilar Magnetometer, arranged to indicate declination.
The Kew Observatory pattern unifilar magnetometer is shown in figs.
In the case of the Kew pattern unifilar the same magnet that is used for the declination is usually employed for determining H, and for the purposes of the vibration experiment it is mounted as for the observation of the magnetic meridian.
- Unifilar Magnetometer, arranged to show deflexion.
2, containing a description of the Kew pattern unifilar magnetometer and detailed instructions for performing the experiments; C. Chree, Phil.
Soc., 18 99, 6 5, p. 375, containing a discussion of the errors to which the Kew unifilar instrument is subject; E.
Fraser, Terrestrial Magnetism, 1901, 6, p. 65, containing a description of a modified Kew pattern unifilar as used in the Indian survey; H.
7, containing a description of a most elaborate unifilar magnetometer with which it is claimed results can be obtained of a very high order of accuracy; K.
The movements of a ship entirely preclude the employment of any instrument in which a magnet suspended by a fibre has any part, so that the unifilar is unsuited for such observations.
In the absence of such a reversible card the index correction must be determined by comparison with a unifilar magnetometer, simultaneous observations being made on shore, and these observations repeated as often as occasion permits.
In each case the magnetograph only records the variation of the element, the absolute values being determined by making observations in the neighbourhood with the unifilar magnetometer and inclinometer.
In the case of the unifilar suspension, the provision of a temperature compensation is not so easy, so that what is generally done is to protect the instrument from temperature variation as much as possible and then to correct the indications so as to allow for the residual changes, a continuous record of the temperature being kept by a recording thermograph attached to the instrument.
Guided by these experiments, Ayrton, Perry and Sumpner constructed an improved unifilar quadrant electrometer which was not only more sensitive than the White pattern, but fulfilled the theoretical law of working.