Sect. 14. 297 for such slow variations the effect described under 2. above will be by far the most predominant. In fact, the equipment provides a record of the time derivative of the element in question. 6) Records of the time derivatives of magnetic elements can, however, be obtained in a more direct way by recording directly the electromotive force (e. m. f. ) induced in suitably disposed coils by the variations of the geomagnetic field 10, 11. When a coil with the total winding area F is placed with its axis in the direction of a geomagnetic force component the variation of magnetic induction B with time will produce an electromotive force 1 dB 1 dH e=- coveu,uuFdT=- 'oV~,uQf1oFdT (i). )) . !. . =_to-13~ d(B/y) (1). )a) v em! d(tjs) in the coil. The e. mJ. may be amplified and recorded by means of an oscillograph or on a tape recorder. but more often the coil is used in connection with an ordinary galvanometer and the scale value of, for instance the dl1/dt record, will then - in the same way as the scale value of the magnet variometer treated in Subsect. IX - be dependent on the damping factor IX and on TO/T, where To and T are the oscillation periods for the undamped galvanometer and for the magnetic pulsation respectively. The amplitude recorded will be proportional to the amplification factor I, obtained from Eq. (1).

This Encyclopedia aims, basically, at summanzmg the wealth of well established facts and outlining the relevant theories in the different branches of physics. With this as goal, the writers were asked to present their specific field in such a way that access is possible to any scientist without special a priori information in that field; the basic concepts of physics are assumed to be known to the reader. The survey given in each paper was also to be long lasting, so that even a few years after publication, each volume would be useful, for example as an introduction for newcomers or as a source of information for workers in a neighbouring field. In the field of geophysics, dealt with in Vols. 47--49 of the Encyclopedia, this task is difficult to achieve because during the last decades there has been a much faster development of basic information and theory than during the decades before. When I came to contribute to this work the famous Julius Bartels, then editor of the geophysical part, told me that Vol. 49 should certainly take into account the results of the "International Geophysical Year" 1957/58 (I. G. Y. ), and that we had better wait until these were accessible than produce a kind of information which might be obsolete in a short time."

83 input to the ions is balanced by a cooling to atomic oxygen, the major neutral constituent, and measurements of the difference between ion and neutral tem perature permit the determination of atomic oxygen concentrations. Using this approach, ALCA YDE et al. 6 have shown from data taken above Saint Santin, France, that the atomic oxygen concentration at 200 km is slightly larger in winter than summer. () The molecular concentrations at heights near 200 km can also be derived from a determination of the ratio of the molecular-ion concentration to elec tron concentration, p (Cox and EVANS 7). It can be shown from the steady state form of the continuity equation for 0+ ions that (20.2) where Q(O+) represents the photoionization coefficient of oxygen atoms, and k and k19 represent the rate coefficients of reactions between 0+ ions and 1S molecular nitrogen Illld oxygen (reactions (12.18) and (12.19)). Then the ratio of atomic oxygen to weighted molecular sum is given by: (l-p)Ne [OJ (20.3) Q(O+) , [kMJ where Ne represents the electron concentration.