OF ARTS AND SCIENCES. 445 



Current, 3.65 vebers. Resistance in circuit, about 10 ohms. Resist- 

 ance of light, 3.3 ohms. With a speed of 1,325, total resistance 9 

 ohms, and current 5.71 vebers, a light varying from GOO to 900 candle- 

 powers was obtained. With a speed of 1,280, resistance 7 ohms, and 

 current 5.20 vebers, the light varied from 650 to 900 candle-powers. 

 Doubtless a much greater light could be obtained with a ditFerent 

 regulator and means of obtaining a high speed. 



The effects of the current were very fine, and have been frequently 

 described in connection with the Wilde, Gramme, and other machines. 

 Thick wires were melted, heavy weights sustained in the air in the 

 interior of large coils, and excellent diamagnetio effects shown. The 

 induced current on breaking the circuit was very severe when taken 

 through the body, and the spark very long and bright. 



The advantages of- this machine are its simplicity, compactness, and 

 small weight, compared with other machines of equal power ; and little 

 or no trouble was experienced from heating with the currents here 

 employed. In conclusion, we wish to express our hearty thanks to 

 I\Ir. Farmer for lending us the machine, and hope that we may be 

 enabled to continue these experiments with this and other machines 

 next year, if we can secure an adequate motor and proper means of 

 measuring power. 



VII. — ANSWEU TO M. JAMIN'S OBJECTIONS TO AMPERE'S 



THEORY. 



Br WiLLiAsi W. Jacques. 



It is the purjiose of this paper to answer some objections which 

 ]\I. Jamin has made to Ampere's theory of magnetism. 



In the Comptes Rendus for Jan. 12, 1874, M. .Jamin published the 

 results of some experiments, in which he obtained the laws of the dis- 

 tribution of magnetism in a soft iron bar which formed the core of two 

 coils by measuring the force necessary to detach an armature when 

 placed at different points along the bar. He gives the etjuations to 

 the curves obtained by sending an electric current through one of the 

 coils, through both coils in the same direction, and through both coils 

 in opposite directions ; and finds that the force necessary to detach the 

 armature at any given point is less when the currents are parallel than 

 when opposed ; from which he draws essentially the following conclu- 

 sions : — 



