638 ELECTRICAL MEASUREMENTS 



of a projecting microscope the images of the strips may be thrown 

 either on a uniformly moving photographic surface, for the pur- 

 pose of obtaining a permanent record, or on a translucent screen 

 for visual observations. 



When permanent records are taken, the photographic surface 

 is caused to move in a direction parallel to the length of the strips, 

 and immediately behind a narrow transverse slit. The projec- 

 tions of the strips cross the slit and on the passage of the alternat- 

 ing current move back and forth along it. The photographic 

 trace is, therefore, a light line on a dark ground. 



For visual observations the image of the strips is focussed on a 

 small ground-glass screen, immediately behind which is placed a 

 stroboscopic disc with narrow radial slits. If the disc be station- 

 ary there will be a bright vertical line of light on the screen, cross- 

 ing which is the dark projection of the strips. The position of 

 this depends on the current strength ; when the disc is rotated, a 

 series of flashes sweep across the screen and as the rotation is 

 synchronous with the current, the curves appear stationary on 

 the screen. 



Theory of the Oscillograph. As the oscillograph is merely a 

 damped galvanometer with a high free rate of vibration, the 

 equation established on page 25 applies. As the current, and con- 

 sequently the deflecting force, may be any function of the time, 

 it must be expressed analytically by a Fourier series. Conse- 

 quently 



nn (1) 



co is 2?r times the fundamental frequency of the current and n is 

 the order of any harmonic. For the fundamental n is 1, for 

 the third harmonic n is 3, and so on. 



If the second member of 1 is zero, and the relative values of 

 P, k and T are such that the motion is oscillatory, 



tf %-(?)' sin [(~f 



K and ip are constants which are determined by the initial 

 conditions. X is the logarithmic decrement and T is the time of 

 a complete vibration (see page 29). 



On account of the damping the transient portion of the deflec- 



