Currents by means of the "PJiaseograph" 



$71 



tbe given curve to represent the characteristic of a carbon 

 filament lamp. How is the resistance now obtained for a 

 certain strength of current (i ) ? The resistance is defined 

 by the equation (1). Consequently the resistance graphically 

 expressed is the tangent of the angle (iOA). If on the other 

 hand the resistance is defined as 



de 

 di y 



another value would be obtained. Hence it follows that 

 the function (1) used for stationary currents in metallic 

 conductors, is of little value in more complicated cases. Its 

 place is taken by the characteristic, where variable currents 

 or inhomogeneous conductors are concerned. If the charac- 

 teristics of two conductors are known, the characteristic 

 of a conductor composed of the two can be constructed from 

 them. Here the following laws for coupling in series and 

 parallel coupling of the two characteristics hold good. 



A. Coupling in Series. — For a certain strength of current (i) 



the two *- values are added. 



B. Parallel Coupling. — For a certain voltage (<?) the two 



2-values are added. 



Fig. 2. 





/ i (l) 



/ 



j£~ 



/ / ^ $fc*U 



Fig. 2 shows the graphical construction of the charac- 

 teristics in question. 



If the voltage (E) and the two characteristics are given 



3S2 



