ALTERNA TI\<;-< ' r/.'A'A'.Y T /.Y.sTtf r .\fENTS 



73 



Its principle is quite simple, being that of a 1)' Arson val galva- 

 nometer (Vol. I, Chap. VII, page 123), as shown in Fig. 70(a). 

 A small phosphor-bronze strip or filament is stretched over two 

 , CC, around a small pulley P and back again. The spring 

 S acting on the pulley keeps the two lengths of the strip in ten- 

 sion. This filament is placed between the poles of a strong elec- 

 tromagnet. When a current flows through the filament, one 

 length of the filament moves outwards and the other inwards. 

 A very small mirror M is cemented across the two lengths of the 

 filament and is given a rocking motion by this movement of the 

 filament. If a beam of light be reflected from this mirror, it 

 will be drawn out into a straight line by the mirror vibration. If 



Current 



(a) 



Vibrating element of oscillograph. 



Method of drawing out vibrating beam 



into a wave. 

 Fio. 70. 



this beam of light be made to strike a rotating mirror, in the man- 

 ner shown in Fig. 70 (6), the rotation of the mirror introduces 

 a time element and the wave is drawn out so that its character- 

 istics are shown. 



The instrument is merely a galvanometer having a single turn 



and a very light moving element. This makes the moment 



of inertia very small. Also, the filament is under considerable 



that its natural frequency of vibration is very high, 



being from iUXX) to 10,000 cycles per second. These character- 



an- necessary in order that the filament may respond 



accurately to the comparatively high frequency variations which 



it is called upon to follow. The moving element is usually 



immersed in oil so that its movement is properly damped and 



the filament is kept cool. 



