384 Intelligence and Miscellaneous Articles. 



further the object he had in view. The Society of Telegraphic 

 Engineers, in accepting the gift of the library, undertook to print 

 the Catalogue, bind the books, and render them accessible to the 

 public — all of which conditions, we believe, have now been fulfilled. 

 Mr. Alfred J. Frost, the Librarian to the Society, has appended 

 a short biographical notice of Sir F. Ronalds*, including some pas- 

 sages from his own account (published in 1823) of the first work- 

 ing electric telegraph, erected by him in 1816. These passages are 

 interesting as illustrations of the discouragement which inventors 

 often have to encounter in bringing their discoveries into practical 

 operation, and which were not wanting in the case of the electric 

 telegraph, and also as historical evidence of the mean opinion held 

 by men of eminence and authority half a century ago about an in- 

 vention which is now found to be one of the most important instru- 

 ments of modern civilization. 



XLV. Intelligence and Miscellaneous Articles. 



ON THE LAW OF ELECTROMAGNETIC MACHINES. 

 BY J. JOUBERT. 

 T ASK of the Academy permission to submit some of the most re- 

 *■ markable results, all verified by experiment, which are deduced 

 from the formula I had the honour of presenting at the last sitting. 

 That formula expresses the law of an important class of magneto- 

 electric machines, characterized by the condition that the variations 

 of the primitive magnetic field follow the law of sines. It gives 



2E 



for the mean intensity of the elementary currents, and 

 tan 2*-0= _ 



for their phase. 



The theory indicates, and experiment verifies in the most rigorous 

 manner, that the maximum value of the electromotive force E 

 during the course of a period is proportional to the velocity ; if its 

 value when the machine makes one revolution per second be called 

 e , and if there are n periods per revolution, we can put, for a given 

 intensity of the field, 



E n = A_. 

 nT 



The formula of the mean intensity then becomes 



2e n 



1 = 



r<7r(R 2 T 2 + 47r 2 IJ 2 y 



* Bringing prominently forward his early work in Telegraphy and 

 Electricity, &c. 



