Intelligence and Miscellaneous Articles. 583 



of magnetization are to be compared with permanent deformations; 

 an analogy which Maxwell had already pointed out. That part of 

 the magnetization which can be applied technically is in the region 

 corresponding to the permanent deformations. It is very probable, 

 and the assumption is confirmed by experiment, that the magneti- 

 zation in very rapid changes of field does not attain this region, 

 while the molecules within the region of the constant /j. can follow 

 far more rapid oscillations than those here used. Further experi- 

 ments, which will perhaps best be made by discharges of condensers, 

 must decide this point. — Wiener Berichte, March 1, 1894. 



RING-ELECTROMAGXET FOR PRODUCING STRONG FIELDS. 



BY H. DU BOIS. 



The large ring employed, made of the best Swedish iron (50 cm. 

 mean diameter, 10 cm. thickness), is slit radially in one place, 

 tangeutially in another, so that the width of the former air-gap 

 may be conveniently varied. It bears 12 coils, each having a 

 resistance of 0*2 ohm and covering 20° (y 1 -) of the circumference ; 

 by means of these a maximum magneto-motive force of 108,000 

 ampere-turns may be applied, requiring about 6-5 horse-power; 

 the highest value of the inductance is about 180 henries, corre- 

 sponding to a " time-ratio " of 180/2*4= 75 seconds; the mean 

 values are of course less. Ballistic measurements are none the 

 less out of the question. 



When provided with flat pole-pieces the apparatus represents 

 the simplest possible type of magnetic circuit, and is therefore 

 well suited for an experimental verification of its laws. By a 

 special method, which is described in extenso, the author was 

 able to verify his form of the theory of magnetic circuits (Phil. 

 Mag. Nov. 1890 ; Wied. Ann. xlvi. p. 491, 1892) with sufficient 

 approximation. The leakage was roughly determined by a com- 

 pass ; its absolute amount was found to reach a maximum for a 

 comparatively low magnetizing current, and to diminish consider- 

 ably as the current was increased. This final decrease of leakage, 

 which of course is more marked still when referred to unit flux 

 of induction through the circuit, was previously found with a 

 smaller ring by H. Lehmann (Phil. Mag. April 1893) and discussed 

 by the author ('The Electrician,' xxix. p. 450, 1892). It w T as 

 explained how this simple fact constitutes an " experimentum 

 crucis" against the fallacy of assuming — arguing from analogy 

 with a voltaic battery immersed in an electrolyte — that leakage 

 must always increase when saturation is being approached. It 

 was also found that leakage is considerably diminished by the 

 ampere-turns near upon the air-gap : the " pole-coils " producing 

 them fulfil the useful function of keeping the flux of induction 

 together, thus preventing the lines from spreadiug. 



The principal object of the apparatus is the production of strong 

 fields by concentrating this flux through properly shaped pole- 

 pieces. A theory of the latter was developed almost simultane- 

 ously (1888) by Stefan and Ewing and Low, in which the 



