100 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



for astronomy. It is to be hoped that when the 200-inch telescope 

 becomes available such a sensitive measuring instrument may be used 

 successfully with a prismatic or grating spectroscope, and with pho- 

 tographic registration, so as actually to secure continuous spectral 

 energy curves of the brighter stars. 



In our attempts to realize the highly sensitive measuring device, 

 we tried for several months to perfect the kampometer, but it did 

 not quite reach our demands. We then turned to the improvement 

 of the galvanometer, hoping to use it to observe the indications of the 

 delicate thermopile such as L. B. Clark now constructs for use in the 

 Division of Eadiation and Organisms. Our intention was to employ 

 with the galvanometer the magnetic shield which was constructed for 

 us about 18 years ago by the late Dr. Elihu Thomson of Lynn, and 

 to insert therein, in highly evacuated space, a galvanometer of the 

 Kelvin suspended magnetic system type. We hoped by high evacu- 

 ation to be able to use a very light system at a time of single swing 

 as high as 10 seconds, and that the sensitiveness would be found 

 nearly proportional to the square of time of swing, even up to this 

 long period. As the magnetic shield is very effective indeed, we at- 

 tempted at first to use a galvanometer of only one pair of coils, with 

 a single group of magnets. But while this arrangement is evidently 

 the most sensitive possible, we found that what was gained in sensi- 

 tiveness was more than lost in instability. Accordingly, we con- 

 structed an astatic system with two opposed groups of magnets sepa- 

 rated but little over 10 millimeters between centers, and two pairs of 

 correspondingly small coils, making a combined resistance of about 

 17 ohms. 



Preliminary trials at Washington indicated a high sensitiveness, 

 but with the mechanical and electrical disturbances unavoidable in 

 a city, we could not tell whether the stability was adequate. To test 

 this question, Messrs. Abbot and Hoover observed, by invitation, with 

 this galvanometer at John A. Koebling's estate in Florida in March 

 1938. They found a 10-second single swing easily practicable; the 

 proportionality of sensitiveness to the square of time of swing nearly 

 followed up to that period, and the stability was so good that readings 

 at a scale distance of 5 meters seemed likely to disclose vibrations only 

 rarely exceeding 1 millimeter in amplitude. 



Tests with the thermopile and candle flame seemed to indicate 

 that the new combination would prove between 10 and 100 times 

 as effective as the combination employed by Dr. Abbot in 1928. Mr. 

 Hoover made a new and better magnetic system after returning 

 from Florida. The excellent showing of the galvanometer is almost 

 wholly due to his work, though based on the extensive researches of 

 Messrs. Abbot and Fowle about the years 1898 to 1900. 



