140 proceedings: philosophical society 



The determination of the pole distance of a very small magnet: J. M. 

 Miller, of the Bureau of Standards. 



If two circular coils, which are linear single turns, are adjusted so 

 that they are coplanar and concentric, and if two currents are sent in 

 opposite sense around the coils, the two magnetic fields will annul each 

 other at the common center if the ratio of the two currents is equal to 

 the ratio of the radii of the two coils. This gives a method of comparing 

 the radii of coils experimentally. In such a comparison the neutraliza- 

 tion of the two fields is usually indicated by a short magnetic needle 

 which is suspended at the common center of the two coils and lies in their 

 common plane. 



On account of the finite length of the magnet an error is introduced 

 into the comparison equal to f K where K is a known function of the 

 pole distance of the magnet and the radii of the coils. If, however, the 

 needle is rotated by torsion or otherwise, out of the plane of the coils, 

 the ratio of the neutralizing currents is changed by a factor -j- K siira 

 where a is the angle through which the magnet has been rotated. Hence 

 by measuring the angle a and also the change in the ratio of the neu- 

 tralizing currents, K can be determined and from this, the pole distance 

 of the magnetic needle can be calculated. 



A piece of steel of only 2.314 mm. length was used as the magnetic 

 needle and the pole distance for this very short magnet was determined 

 to an accuracy of about three per cent. The resulting value for the 

 pole distance was 1.956 mm. 



Kohlrausch's modulus which is the ratio of the pole distance to the 

 total length of the magnet is, therefore, equal to 0.845. Thus in the 

 case of this extremely short magnet, the poles are situated approximately 

 one-twelfth of the total length from the ends, which agree closely with 

 the results obtained by Kohlrausch and others for long magnets. 



R. L. Faris, Secretary. 



THE BOTANICAL SOCIETY OF WASHINGTON 



The 78th meeting was held February 6, 1912. Mr. W, W. Ashe, of 

 the Forest Service, and Dr. H. W. Wollenweber, of the Bureau of 

 Plant Industry, were elected to membership. The following papers 

 were read: 



Conditions favorable to Septoria ly coper sici Speg.: J. B. S. Norton. 



The author gave a brief resume of a series of observations on the de- 

 velopment of Septoria under different conditions influencing growth. 



The relation of soil acidity to plant societies: A. W. Sampson. 



The plant ecologist has long recognized the fact that different soils 

 support different plants. The significance of characteristic plant socie- 

 ties and communities, with respect to the acidity or alkalinity of the 

 soil and its effect on the growth of cultivated plants, was strikingly 

 brought to my attention in a study of the revegetation of depleted 

 grazing lands on the Wallowa National Forest in northeastern Oregon. 



It was found that the soil preference of some of the most promising 



