Phygics. — "Contribution to the knowledge of the string galvano- 

 meter'' Commiinioated by Prof. J. K. A. Wertheim Salomonson. 



(Communicated in the meeting of October 25, 1913). 



I. Intemity of the magnetic field rvith prismatic pole-pieces. Stefan 

 (Wied. Ann. 38, 1889, p. 440) has shown that in an electi'O-magnet 

 with a round core the number of the magnetic lines in the inter- 

 ferricum could considerably be increased, by giving the pole-pieces 

 a conical form with a top-angle of about 110°. In the string galvano- 

 meter prismatic pole-pieces are used. What must be the top-angle 

 in this case in order to obtain a maximal magnetization in the 

 interferricum ? 



Stefan admits in his calculation, which is afterwards also used 

 by P. Weiss (Soc. fr. d. Phys. 1907, p. 132) that the lines of force in 

 the magnetically saturated iron core are parallel to the axis, and 

 that at the surface of the cone free magnetism is extant. We divide 

 the conical surface into a succession of intinitely narrow circular 

 strips standing perpendicularly to the axis. Each point situated on 

 such a strip, exercises on a point placed at the summit of the cone 

 an attraction, inversely proportional to the square of the distance. 

 The axial component of this force may be represented by an ex- 



pression proportional to -— m which / represents the distance, and 



r 



a half the topangle. If we put / =: in which /• indicates the 



^ sin a 



radius of the circular slri]), then the axial force becomes proportional 



cos a sin' a 

 to 



This expression has a maximum for cos « = 1/ y or « = 



about 55°. 



What holds for one detinite point of the circular strip, holds for 

 every point of any other circular strip situated on the conical surface. 

 Consequently the most favourable top-angle for the cone amounts to 

 about 110°. 



We can apply a similar reasoning in the case of prismatical pole- 

 pieces. In this case we divide the prismatic surface into infinitely 

 narrow strips parallel to the rib. 



We calculate in the tirst place the attraction, exercised by an infini. 



