34 ELECTRIC LIGHTING. 



Internal Resistance of the Machine. On testing by the 

 method of Wheatstone's bridge, the resistances of the ma- 

 chine experimented with were found to be : 



ohms. 

 Total resistance of the machine before working ... ... ... i'i35 



Resistance of the coil when warm, after having been working 



some time in short circuit 075 



Resistance of the electro-magnets under the same conditions ... 072 

 Total resistance of the machine when warm ... ... ... i'47 



These figures show that the internal resistance of the ma- 

 chine varies within very narrow limits, and that these varia- 

 tions are due to the heating of the wire, the resistance of 

 which increases with the temperature. 



Electro-motivs Force. Curve I. of the diagram, given in 

 Fig. 74, shows the variation of the electro-motive force when 

 the external resistance increases. The total resistances are 

 referred to the axis of the abscissae, on a scale of T yhs of 

 a centimetre per ohm ; the electro-motive forces are referred 

 to the ordinates on a scale of T Vths of a millimetre per volt. 

 When the total resistance is more than four times the in- 

 ternal resistance, it will be seen that the electro-motive force 

 is nearly constant and very feeble. This is due to the fact 

 that the induction on the coil is produced only by the re- 

 sidual magnetism of the electro-magnets. Then this electro- 

 motive force increases very rapidly between 6 ohms and 4 

 ohms of total resistance, and it reaches a value of 107 volts, 

 varying then very little. This is caused by the electro- 

 magnets being magnetized to saturation ; the magnetic field 

 remains constant, and as besides the speed of the induced 

 system is constant, the electro-motive force, which is pro- 

 portional to these two quantities, can vary only by the 

 heating of the wire. 



Intensity of the Current. The intensity of the current, 

 expressed in webers, is represented by the curve II. of the 

 diagram on a scale of y^ths of a millimetre per weber. 



