332 CARNEGIE INSTITUTION OF WASHINGTON. 



On certain matters relating to the theory of atmospheric-electric measurements. W. F. G. 

 Swann. Terr. Mag., vol. 19 (Dec. 1914). 



The earthed portions of pieces of apparatus exposed to the atmospheric 

 potential gradient have to take a negative charge in order that their potential 

 may remain zero. The effect of the charge in modifying the measurements is 

 discussed for various cases. In the Ebert ion counter the effect is to decrease 

 the measured ionic density in the case of the negative ions, while in the case 

 of the positive ions the results are unaffected, though the paths of the ions are 

 of course influenced by the charge. It is shown that the effect is expressible 

 very simply in terms of the charge induced by the potential gradient on the 

 inside of the opening where the air enters. By measuring this charge it is 

 possible (knowing the rate of the air-flow) to estimate the order of magnitude 

 of the effect. The effect is greatest of course when the apparatus is mounted 

 on a tall, slender support. It was found by measurement that it might easily 

 amount to 20 per cent of the value of the quantity measured. The analogous 

 effect is discussed for the case of the Gerdien conductivity apparatus, and it is 

 shown that even in the case of the negative ions no error is introduced, provided 

 that the potential of the central conductor is not too high. In fact, the effect of 

 the potential gradient is simply to lower the value of the maximum potential 

 to which it is allowable to charge the central system. In the case of the con- 

 ductivity of the positive ions, while the ionic paths are influenced, neither the 

 conductivity nor the maximum potential allowable are influenced on the whole. 



The effect of making measurements of conductivity on the top of a high 

 tower is discussed in the above light and it is shown that if suitable precautions 

 are taken, the proper conductivity will be obtained for the air at that altitude, 

 in spite of the influence of the charge on the tower. There is an advantage in 

 making measurements on the air far removed from the ground, since the 

 number of ions present are then uninfluenced by the effects due to the poten- 

 tial gradient referred to in the first abstract. 



The question of shielding the instruments to protect them from the potential 

 gradient is discussed. It is only in the case of the Ebert apparatus that 

 shielding of this kind is necessary. It is important to ascertain, however, 

 whether the avoidance of charge on the earthed portions of the apparatus by 

 shielding is not counteracted by the effect of the charge on the shield. It is 

 shown that a properly arranged shield is on the whole effective. 



Some points with regard to the variation of the specific magnetization of a substance with 

 temperature. W. F. G. Swann. Physic. Rev., ser. 2, vol. 3, p. 485 (1914). 



According to the theory of Weiss, the molecular magnetic theory of a solid 

 may be treated on the lines of that for a gas, provided that the applied field H 

 is replaced by H-\-Hm, where Hm is a field which represents the effects of the 

 molecular actions. Hm is assumed to be of the form NI, where / is the inten- 

 sity of magnetization and A'' is a constant for the substance. 



Weiss has shown that in order to account for the variations of the specific 

 magnetization with temperature above the Curie point, it is necessary to 

 assume that the number of magnetons in a molecule changes abruptly at 

 certain temperatures. In the present communication it is shown that this is 

 not all that is necessary to account for the facts, but that the constant N must 

 also vary abruptly with the temperature and in some cases must even acquire 

 a negative value. 



A change in the number of magnetons is usually not accompanied by an 

 abrupt change in the coefficient of magnetization, which fact seems to indicate 

 a tendency on the part of the substance to make a change in the number of 

 magnetons only when the temperature conditions are such that it can do so 

 without altering that part of the energy which is purely magnetic. 



