298 Intelligence and Miscellaneous Articles. 



Bpecific gravity amounts to 1-2. The fragments of the mineral 



are stuck upon the ball and the whole put into a solution of iodide 

 of mercury in potassium iodide. The latter is then diluted till the 

 wax ball just floats in it. If P. V, D and p, v, d are respectively 

 the weight, volume, and density of the float and of the substance to 

 be investigated, and A the specific gravity of the liquid, then 



d= P* . 

 P+pAV 



Wiedemann's Beibliitter, 1880, ]S r o. 7, p. 497. 



ON THE LAW OF MAGNETOELECTRIC MACHINES. BY J. JOUBERT. 

 I recently had the honour of communicating to the Academy* 

 the experimental methods which I employ in order to study 

 the laws of the alteimating currents used for the production of the 

 electric light. The application of those methods to Siemens's 

 alternate-current machine has shown me that the mean intensity 

 of the current given by that machine is very accurately represented 



by the formula I=— — r-r, in which E is the total resistance 



of the circuit, m a constant depending only on the velocity and 

 varying in the inverse ratio of the duration T of the period ; and 

 C is another constant, equal to the quotient by \/2 of the maximum 

 value of the electromotive force of the machine working with open 

 circuit, measured directly. 



The simplicity of the result, and the complete concordance of 

 the experiments with the formulae, made me think I had before me 

 not merely an empiric formula, but the expression itself of the law 

 of the phenomenon ; and I was led to try whether the theory could 

 not conduct me back to that formula. 



Let us suppose that the motion of the machine is uniform. Let 

 E be the value, at a given moment, of the electromotive force 

 resulting from the primitive magnetic field — that is to say, of the 

 field as it exists when the induced system is at rest ; and let I be 

 the quantity of electricity set in motion during the time dt, starting 

 from that moment. The electromagnetic work is equal toEL#, and 

 is found again in the thermal work of the current I 2 Hdt and in the 

 work of the inverse electromotive forces which spring from the re- 

 actions of the various parts of the machine. Experience shows that 

 the reactions upon the inducing electromagnets are negligible ; for 

 the current of the excitatrix, measured by an extremely sensitive 

 galvanometer, shows no variation when the induced circuit is closed 

 or when it is opened ; therefore the reactions are reduced to the 

 induction of the current upon itself. If we represent by U the 

 flow of force emanating from the induced system when it is traversed 



by the unit of current, and, consequently, by — what is called the 

 * Comptes Re?idus, July 26, 1880. 



