JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. io MAY 19, 1920 No. 10 



PHYSICS. — A simple substitute for a cathetometer } J. B. Fer- 

 guson, New York City. 



A cathetometer is often used, especially by investigators of 

 problems involving gases, when a pressure reading is desired 

 with an accuracy of o.i mm. or better. This instrument is, 

 however, very cumbersome and also expensive. It may be 

 replaced to advantage by a "micrometer depth gauge," which is 

 an inexpensive- machinists' tool of small size capable of giving 

 readings with an accuracy of o.oi mm. or 0.0005 inch. 



These gauges usually consist of a vertical rod, with a sliding 

 head mounted perpendicular to the rod and provided with a 

 micrometer or a vernier. The head has a polished surface about 

 ^/o inch (13 mm.) in width, which is sufficient to enable one to 

 make an accurate setting by sighting across it against a mercury 

 meniscus. 



To illustrate the possibilities of such an instrument, one was 

 set up in a retort stand with the head opposite the two surfaces 

 of mercury in a partly-filled U-tube, and various observers 

 determined by it the position of one mercury meniscus. Read- 

 ings were made first with the gauge in front and then behind 

 the U-tube. The results are given in table i . This instrument 

 was graduated in inches, but instruments graduated in milli- 

 meters are also obtainable. 



' Received April 6, 1920. 



- A four-inch instrument would cost not over ten dollars. 



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