PROCEEDINGS OF THE ACADEMY AND AFFILIATED 



vSOCIETIEvS 



PHILOSOPHICAI. SOCIETY 



820TH MEETING 



The 820th meeting was held at the Cosmos Chib, May 24, 19 19, 

 with President Humphreys in the chair and 34 persons present. The 

 minutes of the 819th meeting were read in abstract and approved. 



The first paper, by Messrs. H. E. Curtis and R. C. Duncan, on 

 Measurements of short time intervals, was presented by Mr. Duncan. 



Early in the war, at the request of the Navy Department, the Bureau 

 of Standards undertook the investigation of certain factors which had 

 to do with the firing of the 14-inch Naval guns. It was desired to 

 measure a number of time intervals into which the total firing time 

 of the gun may be divided and also to study the motion of the gun as a 

 function of time. Both of these problems required the measurement 

 of short time intervals with considerable accuracy. 



The General Electric oscillograph was chosen as the apparatus for 

 recording the events which were to be studied. A special timing 

 system was installed in the oscillograph, which ruled on the film an 

 equal interval time scale, consisting of sharp lines running across the 

 film. By means of a proper optical system, light from the arc used 

 to illuminate the mirrors of the oscillograph was caused to fall on the 

 film after passing through a narrow slot, which was open only for very 

 short times at regular intervals. This slot was made by fastening a 

 slotted aluminum vane on each prong of a tuning fork and adjusting 

 the vanes so that when the fork is at rest the slots will allow the light 

 to pass through. As the fork vibrates, these slots are opened twice 

 each vibration. A 500-cycle fork is used to rule the film in thousandths 

 of a second, while the 50-cycle fork rules the film in hundredths of a 

 second. 



Considerable trouble was experienced in driving the 500-cycle fork 

 at sufficient amplitude. Satisfactory results, however, were finally 

 obtained by driving 500-cycle forks by means of a loo-cycle master 

 fork. In this way the electro-magnets of the 500-cycle forks were 

 energized once every five vibrations and by careful tuning it has been 

 possible to get a double amplitude of nearly 2 mm. This is important, 

 as the sharpness of the timing lines depends upon the amplitude of the 

 fork vibrations. In order to eliminate any possible errors due to the 

 master fork, a relay is so arranged that as the exposure is made the 



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