2l6 THE SCIENTIFIC PAPERS OF 



developed in the electro-magnets. This force depends upon the 

 amount of current passing through the coils of the magnets, 

 which in its turn is dependent in an inverse ratio upon the resis- 

 tance in the outer circuit. If from some accidental cause the 

 external resistance is increased, the electro-motive force of the 

 machine, instead of rising to overcome the obstruction, diminishes, 

 and thus aggravates the resulting disturbance. If, on the other 

 hand, the resistance of the outer circuit diminishes, as in the case 

 when the carbons of an electric regulator touch one another, the 

 electro-magnets are immediately excited to a maximum, and the 

 electro-motive force of the machine is increased. The power 

 absorbed and its equivalent, the heat generated in the circuit, is 

 equal to the square of the electro-motive force divided by the 

 resistance ; hence the work demanded from the engine will be 

 greatly increased, the machine may be dangerously overheated, 

 and powerful sparks may injure the commutator. It is chiefly 

 owing to this instability of the dynamo-electric current that its 

 application to electric illumination has been retarded, and that 

 magneto-electric machines and machines producing alternating 

 currents have been again used, although they are inferior to 

 the dynamo machine in the current energy produced for a given 

 expenditure of mechanical energy. 



The properties of dynamo-electric machines have been examined 

 by several observers. Messrs. Houston and Thomson (Franklin 

 Institute) compared the efficiency of the Gramme, Brush, and 

 Wallace Farmer machines. Dr. Hopkinson (Institution of 

 Mechanical Engineers, 25th April, 1879) examined a medium- 

 sized Siemens machine, determined its efficiency, and expressed 

 the electro-motive force as a function of the current. Herren 

 Mayer and Auerbach (Wiedemann's " Annalen," November, 1879) 

 experimented on a Gramme machine. M. Mascart has experi- 

 mented on the Gramme machine, and Mr. Schwendler on both 

 Gramme and Siemens machines. 



The radical defect of the dynamo machine of ordinary construc- 

 tion, may be inferred from the results of these experiments. The 

 remedy has, however, been in our hands from the time of the first 

 announcement of the principle of these machines before the Royal 

 Society, when Sir Charles Wheatstone pointed out that " a very 

 remarkable increase of all the effects, accompanied by a diminution 



