Study of Variable Currents by " ' Phaseograph" 969 



If the eye is slightly nearer the object-glass than the 

 position E', and directed to A, both beams are seen, but if 

 the eyeball is turned to the right, instead of P (which appears 

 to the right always) becoming more plainly seen, it 

 disappears. 



And finally, if the eye is just on the side of E' remote 

 from the object-glass, and directed to A, nothing is seen. 

 Bat if the eye is turned to the right, P', towards the left, 

 becomes visible. 



These facts, obrious enough, are pointed out as assisting 

 very considerably the process of adjustment. If difficulty is 

 experienced in keeping the eye fixed upon A, a small pinhole 

 may be made in the screen at that point. The beam of 

 light through it will, during the processes described, be 

 always visible. 



The values of the diameter of the apparent pupil, under 

 the circumstances indicated, in the writer's own case, have 

 been found to vary from 6*74 to 7'20 millimetres, the higher 

 values being less frequent. He cannot but look upon the 

 value £ of an inch (8*47 millimetres), quoted occasionally in 

 text- books, as extremely abnormal. 



CIX. 7 he Study of Variable Currents by means of the 

 "Phaseograph" By Manne Siegbahn*. 



[Plate XIX.] 



Introduction. 



IN order to characterize the electric state of a conductor 

 through which an electric current passes, the two 

 variables, strength of current (i) and voltage (e), may con- 

 veniently be made use of. As long as we have to deal with 

 metallic conductors and constant currents, there is between 

 these two variables the simple relation : 



- = const. = m, (1) 



where the constant m signifies the ohmic resistance in the 

 conductor concerned. 



In the general case, variable currents and conductors of 

 different kinds, the connexion between the variables is more 

 complicated. In order to obtain a survey of matters in these 



* Communicated by the Author. 



Phil. Mag. S. 6. Vol. 20. No. 120. Bee. 1910. 3 S 



