418 



KENNELLY, TAYLOR— PROPERTIES OF 



tuating current. If the alternating current has a lower frequency 

 than the particular resonant frequency to which the instrument is 

 adjusted, the mirror displacements will be substantially in phase 

 with the current; whereas if the A.C. frequency is higher than the 





Vrbrctiiori, 



Ghost clue ^o defect m 



Oscillograph Mirrar. 



LiS5djou5 ficjure for Diaphr«<jfn ff) 

 ^^hokvin^ Phase Difference ir\ Vibrution 



Fig. 2. 



instrument's resonant frequency, the mirror displacements will be 

 in opposite phase to the current. There was therefore no prac- 

 tical difficulty in ensuring one or the other of these two conditions, 

 as might be desired, for impressed alternating-current frequencies 

 up to 1,500 '— ', the Duddell galvanometer employed having a range 

 from 100 to 2,000 '~'. 



Fig. 2 is a photographic record of a Lissajous figure, obtained 

 on the screen S of Fig. i, for a particular case. These test records 

 were photographed and analyzed. The well-known analysis is re- 

 produced in Fig. 3. The Lissajous ellipse ABCD about the center 

 o and coordinate axes Xx and Yy, is supposed to be described in the 

 direction of the arrows. Corresponding i)oints on the circles 

 A^B^C-^D.^ and AoBX.,D^, are projections from the ellipse on the Y 

 and X axes respectively. The phase difference between the simple 

 harmonic motions of Fy and Xx is any one of the four equal angles 

 a,, cto, a:t, and a^■ Owing to imperfections in the photographic 



