1442 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



Part II — Displacement and Velocity 

 Measuring System 



The Optical System} 



A block diagram of the displacement and velocity measuring system 

 was shown as Fig. 4. As part of this figure, a geometric schematic of the 

 optical part of the system is shown. Most of the features of the optical 

 system are due to the advice and assistance of associates in these lab- 

 oratories who were connected with the sound motion picture develop- 

 ment. The fundamental design is similar to the film reproducing system 

 with certain necessary changes in dimensions. 



Referring to Fig. 4, the elements are the lamp, condensing lens, a slit, 

 objective lenses, a vane on the part whose motion is to be studied, and 

 a photocell. The lamp and the condensing lens are the same as are used 

 in the film reproducer. The slit is the same except that its width has been 

 increased to 0.005'' and a feature added permitting its length to be ad- 

 justed from zero to about 0.1''. The objective lenses are inexpensive 

 Bausch and Lomb achromatic lenses. The second objective lens is inter- 

 changeable for different focal lengths, and may be moved back and forth 

 in the supporting tube for precisely focusing the image of the slit on the 

 moving vane. 



The three lenses and slit are mounted in a tube fastened to the lamp 

 housing which in turn is supported by adjustable supports mounted on 

 a vertical stand. To the lamp housing support is also fastened the photo- 

 cell container with its first amplifier tube. 



A permalloy shield surrounds the photocell except for an opening for 

 access of light which passes by the moving vane. The shield is provided 

 to prevent the changing stray magnetic field from the nearby electro- 

 magnet under test from affecting the photocell current. This system can 

 be moved in any of three directions for lining up with the vanes of the 

 device being tested. The relay or structure being measured is not fastened 

 to the optical system support, but is secured to another appropriate 

 stand resting on the same laboratory bench. 



This system may seem more complicated than necessary at first sight, 

 but its advantage is that it provides a rectangular beam of light of con- 

 stant width and adjusta})le length at the point being measured. The 

 necressity of numerous light shields and screens is done away with, al- 

 though it is advisable to throw a black cloth over the whole apparatus 

 when once adjusted to keep out the overhead illumination. 



The reason for this last is of interest. The fluxmeter timing is controlled 

 by a stable oscillator. The 60-cycle power for the overhead lights is also 



