564 Dr. S.' P. Thompson and Miles Walker on the 



saturation. At this point there will necessarily be as much 

 of the acid at first combined with the base, now free in the 

 solution, as corresponds to one molecule of a bibasic acid, 

 that is two of a monobasic acid, half a molecule of a quadri- 

 basic acid, &c. From this we can deduce the exact nature of 

 the resulting equilibrium. 



2. That a series of equilibria thus obtained with different 

 salts enables us to determine the comparative strength of the 

 affinities of the acids of those salts. 



3. That the fact, already proved in other ways, that even 

 small quantities of weak acids, added to sulphates, will set 

 free a certain quantity of sulphuric acid can, by means here 

 given, be for the first time rendered visible to the eye by a 

 well-marked chemical reaction. 



LVI. On the Design and Winding of Alternate- Current 

 Electromagnets. By Silvanus P. Thompson, D.Sc, 

 F.R.S., and Miles Walker*. 



1. O designing electromagnets for use with alternate 

 JL currents various considerations enter which do not 

 enter into the design of electromagnets for use with continuous 

 currents. Chief of these considerations are the self-inductive 

 action of the windings, and the frequency of the alternation 

 of the current. 



As with continuous- current electromagnets, so with alter- 

 nate-current magnets, the degree to which magnetization is 

 carried in the magnetic circuit of given configuration depends 

 upon the number of ampere- turns of the excitation. The 

 methods of predetermining the number needed to produce any 

 desired degree of magnetization are too well known to need 

 any reference. Our present aim is to show how to determine 

 the winding which will, under given circumstances as to 

 frequency, voltage, &c, produce any desired number of 

 ampere-turns. 



2. As a preliminary, however, it is needful to touch upon 

 the point whether an electromagnet (properly laminated, it is 

 understood) will give an equal pull, when excited with an 

 equal number of ampere-turns of alternating current. 



Since the pull of an electromagnet upon its armature 

 depends upon the normal tension along the magnetic field 

 integrated over the area of the pole-face, and this tension 



* Communicated by the Physical Society : read April 27, 1894. 



