320 



EXPLORATION GEOPHYSICS 



In the photographic recording balance a dot is made on the photo- 

 graphic plate to record the rest position of the beam, as has been previously- 

 shown. (Figures 161 and 183.) A fixed mirror in the optical system of 

 the instrument deflects a portion of the light beam, to give a fixed spot, or 



Fig. 180.— Optical system for measuring the deflec- 

 tion of a torsion balance beam. f= focal length of lens 

 expressed in scale divisions. 



PHOTO PLATE 



reference mark, on the sensitized plate. This occurs simultaneously with 

 the recording of the deflection. 



The distance n from the fixed dot to the dot representing the deflected 

 position of the beam is measured on the photographic plate with an accur- 

 ately-made glass scaling-plate. The scaling plate is graduated in j^ mm. 

 divisions. The torsionless position of the balance (which has special sig- 

 nificance, as will be shown) or the position of the beam when it is not 



deflected would have a distance 

 Ho scale divisions from the fixed 

 dot on the photo plate. In photo- 

 graphic recording the scaling plate 

 is substituted for the scale used 

 (n) 1 """""Z ;! in visual observation of beam 



deflections. 



If, then, the distance of the 

 dot representing a beam deflection 

 a is given by the distance of w 

 scale divisions as scaled from the 

 fixed or reference dot of the pho- 

 tographic plate, and if / is ex- 

 pressed in ^ mm. scale divisions, 

 it follows that : 



At MIRROR 



n — Ho = 2 a f 



(72) 



PART (a) IS OPTICALLY EQUAL TO (b) 



Fig. 181. — Part (a), optical arrangements in a 

 torsion balance for photographic recording of beam 

 deflections. Part (b) shows simplified diagram of 

 the relation between light, lens and plate, f = focal 

 length of lens, expressed in scale divisions. 



In the Z-beam torsion balance, 

 as indicated in Figure 181, the 

 balance mirror is set at an angle 

 of 45° to the axis of the beam. 



