Temperature Effects.—All except D magnetographs suffer from temperature effects. A rise of temperature causes a fall in M, and so has the same effect on MH, for example, as a fall in H. There is a consequent decline in the H ordinate and similarly with V. Temperature again alters the torsion in a unifilar H suspension, and the position of the centre of gravity in the V magnet. There are various compensation devices, e.g., in the case of V a horizontal zinc rod whose excess of expansion over steel helps to neutralize changes in M by changes in the centre of gravity. In the Eschenhagen pattern changes produced in an auxiliary field due to auxiliary magnets neutralize the direct effects of temperature on the principal magnet system. Compensa tion is usually imperfect, and corrections to the curve ordinate are calculated with the aid of a continuous thermograph record. The ideal thing is the use of a constant temperature room.
Deflection Scale.—In the case of D, when torsion is negligible, as is usual in the Kew pattern, the scale value is invariable, de pending only on the optical distance between the magnet mirror and the paper; the usual value is about 1' per i mm. i.e., I min ute (I') per i millimetre (I mm.). When torsion is not negligible, its effect must be found by observing the change in the D ordinate produced by turning the torsion head through a given angle, and a corresponding correction must be applied to the scale value cal culated from the optical measurements.
In the case of H, when the suspension is bifilar, the scale value can be altered by altering the distance apart of the two arms of the suspension. Similarly in the case of V the height of the centre of gravity, and so the sensitiveness, can be altered by means of a vertical screw. In these cases a pre-arranged scale value can be secured and maintained. Auxiliary magnets are sometimes used, the scale values altering with their position. Regular redeterminations of scale value are necessary in all force magnetographs. At some stations the magnet is at the centre' of a large coil. A measured electric current produces a known change of magnetic field, and the scale value is calculated from the change of curve ordinate. In other cases an auxiliary magnet of known moment is used to deflect the H and V magnets. In Broun's method a knowledge of the moment of the deflecting magnet is unnecessary. The D magnet is deflected as well as the H and V magnets, the distance between the deflecting and deflected magnets and their relative positions being the same. The scale values are inversely as the changes of ordinate, the deflecting forces being the same, and the D scale value (in terms of force) can be calculated using an approximate value of H. Broun's method really assumes the D, H and V magnets closely alike, as in the Kew pattern, or else the deflection distances large.
scale values, per I mm., of 1' in D and 5 y in H and V are con sistent, especially as natural changes in H and in D (in terms of force) are very similar. But in India, where H may exceed 0.39 and D is a very quiet element, 0.5' per 1 mm. would be more suitable. In high latitudes 2' or even 5' per 1 mm. may be de sirable. In fact a D magnetograph is then so troublesome that it may be better to measure the changes of force in and per pendicular to the geographical meridian. For some purposes open scale magnetographs, say i7 or 27 per 1 mm., have advantages, but there is great risk of loss of trace at disturbed times. This risk can be reduced by the attachment to the magnet of two mirrors inclined at a small angle. This double mirror arrange ment is usually carried by one at least of the Eschenhagen magnets. Eschenhagen also made use of a mirror inclined to the horizon, to give a supplementary trace of much lower sensitive ness. In really big magnetic storms a multi-mirror Eschenhagen magnetograph sheet may have a great mix up of traces. It is simpler, if funds permit, to run two magnetographs, one of high, the other of low sensitiveness.