168 



McCLELLAN— USE OF OSCILLOGRAPH. 



[April 



taneously. The free period is approximately one ten-thousandth 

 of a second, undamped. There are three mirrors, one fastened on 

 each coil, and one fixed in the center, to give a zero line. The 

 maximum current used is about one tenth ampere. 



To obtain a curve, it is necessary to provide uniform motion 

 perpendicular to the motion of the mirrors. In this instrument it 

 is accomplished by means of a falling photographic plate. This 



motion is uniformly accelerated, but the error in the length of the 

 plate is very slight, though measurable. The error amounts some- 

 times to about a half per cent, of the wave length. The arrange- 

 ment can be understood from Fig. 2. The galvanometer M is 

 placed in a camera as shown. This is provided with a slit in the 

 end, through which parallel light is sent. The ribbon of light 



