306 ELECTRIC LIGHTING. 



order to reduce to French units or kilogrammetres we must 

 know that the kilogrammetre equals 98*1 meg-ergs. 



Curve III. of the diagram shows that the work trans- 

 formed into electricity, which is very small when the total 

 resistance exceeds 6 ohms, increases afterwards rapidly and 

 regularly as the resistance diminishes. The scale of curves 

 III. and IV. is T 7 oths of a millimetre for 2 kilogrammetres. 



Utilizable Work in the External Circuit. The current 

 produced by a magneto-electric machine is divided into two 

 parts : the internal work which heats the wire of the coil and 

 the electro magnets, and which cannot be utilized, and the 

 available work produced in the external circuit, and which 

 may be employed either to heat a wire, as in our experi- 

 ments, or to produce different effects. 



The utilizable work, represented by curve IV. in the 

 diagram, after having been very small, increases as the total 

 resistance diminishes. It reaches its maximum when the 

 external resistance is equal to the internal resistance of the 

 machine, and afterwards diminishes to nothing when the 

 machine is arranged in short circuit. In this case the whole 

 of the work supplied by the motor is transformed into in- 

 ternal work, the machine becomes greatly heated, the brushes 

 burn, and the insulation of the wire may even be damaged. 



Duty. Duty in its general sense is the ratio between the 

 work expended and the work utilized If only the work 

 transformed into electricity is taken into account, as we shall 

 do here, by neglecting the passive resistances and the fric- 

 tion of the parts, the duty is the ratio between the total work 

 transformed into electricity (curve III.), and the work utili- 

 zable in the external circuit (curve IV). This ratio, always 

 less than I., is represented by curve V. in the diagram. It 

 will be seen that this ratio increases with the resistance, and 

 tends towards I. for an infinite resistance, in which case 

 there is no longer any current. This assertion seems, in 



