DETERMINATION OF WAVE FORM 



631 



pole pieces, P, are inserted so that the middle of the iron strip is 

 in a very narrow air gap. Midway between the bridge pieces, 

 and behind a small window, the minute mirror, m, is attached 

 to the strip. This movable system fits between the laminated 

 poles of a powerful electromagnet which conforms to the out- 

 side of the tube. The middle part of the soft iron strip thus 

 becomes, in effect, a polarized needle. The required damping 

 is obtained by filling the tube with a transparent oil of the proper 

 viscosity. 



JU 



Fio. 3!).-,. Galvanometer for Blondel moving-needle oscillograph. 



The current whose wave form is to be determined is led through 

 two coils placed with their common axis perpendicular to t 

 magnetic axis of the needle. On the passage of the current 

 needle will deflect like that of an ordinary galvanometer and 1 

 spot of light will be moved across the photographic surf; 



This type of instrument may be given a very high free period 

 of vibration, but.has the disadvantage that its self-mduction , 

 comparatively large. 



