978 Mr. M. Siegbahn on the Study of Variable 



and capacity. Through one wire passes then a current 



i = i $hi(ot, (16) 



and through the other 



e± e sin (©* + <£) (17) 



By the combined action of these oscillations the luminous 

 point describes a figure the equation of which is obtained 

 from (16) and (17) by the elimination of t : 



re' 



cos ^>=sin 2 (f>. 



(18) 



In the general case the luminous point, consequently, 

 describes an ellipse. What is of interest in this case is the 

 phase-difference between the two currents. It is especially 

 interesting as by the phase difference can he calculated 

 e.g. the self-induction of a bobbin. For the phase-difference 

 between current and voltage we have 



n </> = 



2ttL 



WT 



(19) 



T, period ; 



L, -elf-induction coefficient ; 



ay, ohmic resistance. 



I will now show m very simple way of calculating tl. 

 ph.-ise-difl'erence from the registered ellipse. 



By letting the two currents (e and i) register one at a 

 time the resistance is altered, till the same amplitudes are 

 obtained, 



h = e (20) 



