Intelligence and Miscellaneous Articles. 323 



attains its greatest value if the distance of the pole-surfaces a 

 satisfies equation 



n% + 1 



in which d is the diameter of the perforation, n the number which 

 indicates by how much the diameter of the iron core is greater than 

 that of the perforation. 



The perforation of the iron core always produces a great defor- 

 mation of the magnetic field. The intensity decreases considerably 

 from the centre towards the end of the field. Between plane pole- 

 pieces the force at the end of the field is smaller than the half of 

 the force in the centre. Iron cores with conical ends of the form 

 just prescribed have the advantage of a greater intensity in the 

 centre, and at the same time a smaller decrease of the force towards 

 the end of the field.— Wiener Berichte, Feb. 9, 1888. 



EXPERIMENTS ON THAT FOEM OF THE DISCHAEGE OF 



ELECTEICITY KNOWN AS ST. ELMO'S FIEE. 



BY A. V. OBERMAYEB. 



The experiments described in the paper show that the positive 

 brushes are reddish white in the stem, with violet radiating branches. 

 The stems which project into the brush are at angles which may 

 reach 90°, and the rays may attain a length of 6 to 7 centim. 

 Further, that the negative brushes which verge into violet are always 

 so fine that individual rays can no longer be distinguished, that 

 these brushes standing on a luminous point show an angle of aper- 

 ture of less than 90°, and that the length of the brush is always 

 under 1 centim. 



In discharges from positive surfaces, for instance from clothes, 

 these appear covered with fine rays of a length up to 3 centim., 

 which are lite the hairs of a fur. 



Discharges from similar negative surfaces consist of a phospho- 

 rescent glow-light which is in continual motion, and is traversed by 

 dark spaces. 



The falls in potential necessary for the production of such a 

 brush-discharge decreases with the distance through which the dis- 

 charge takes place. In volts per centimetre this fall was approxi- 

 mately : 



At 5 centim. distance 6000 volts per centim. 

 15 „ „ 4000 



30 „ „ 1600 



