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BELL SYSTEM TECHNICAL JOURNAL 



our knowledge there are only two in this country at present, the other 

 one being in the Bureau of Standards. 



The light is furnished by the helium tube 'M." The particular 

 wave length to be used is selected by rotating a glass prism inside the 

 case by means of the cylinder "5" graduated to read the wave length 

 directly. The gage block to be measured is located at "C" and the 

 interference bands are observed through the eyepiece "£." 



Fig. 28 — Laboratory interferometer 



To make a direct measurement of the length of a gage, observations 

 are made using five wave lengths, and by computing the results ob- 

 tained the length may be determined to an accuracy of about two 

 millionths of an inch. In working with this degree of precision, the 

 instrument is used in a constant temperature room and corrections 

 made for temperature, humidity, and barometric pressure. 



Conclusion 



Tool making in all its branches as carried on in the tool rooms of 

 the Western Electric Company is a comprehensive subject, regarding 

 which several volumes might be written if covered in detail. In the 

 foregoing description an effort has been made to give briefly, and by 

 considering only one branch of the work, a general picture of the 

 high grade workmanship required and some of the equipment and 

 instruments employed. Similar precision is required on many classes 



