Periods And Logarithmic Decrement Of The Gravitation Needle Under High Exhaustion

VOL,. 8, 1922 PHYSICS: C. BAR US 63 The values of the radiation potentials for strontium and barium have not been directly determined, but Mohler, Foote and Megers (Sci. Paper, Bureau of Standards, No. 403, 1920) give estimates from the wave-lengths of the tail lines of the proper spectral series for these elements. Hydrogen is added to the table, although not given by Eve. An inspection of the table brings out the following: For Group I, the degree of constancy of the products is nearly the same, being slightly in favor of the use of I rather than (I - R). Group II A, shows, on the other hand, that (I - R) gives a better agreement than I. This is still more marked in Group II B. Group V A is considerably more favorable to (I - R). In case of the inert gases the product using I alone does not hold at all, while the product (I - R) gives nearly a constant. The product using (I - R) places hydrogen in good agreement with the first Group. Mercury, which has some of the physical qualities of the inert gases, has a product about equal to that for those gases. In case of the whole table, going from Group to Group, the products using (I - R) are much more constant than is the case with I alone. PERIODS AND LOGARITHMIC DECREMENT OF THE GRAVITATION NEEDLE UNDER HIGH EXHA USTION* By C. BARUS DEPARTMENT OF PHYSICS, BROWN UNIVZRSITY Communicated, February 4, 1922 The Deflections.-After a search for a finer quartz fibre than was used in my last paper, one was found giving a double deflection of Ay = 13.42 + .03 cm. as compared witlh the former 2.67 cm., for the same case and needle. In spite of this astonishing sensitivity and otherwise admirable behavior of the apparatus, the new result for Ay from 30 successive night observations came out relatively less accurate than the former. At the same time, the observations on any single night (details must be omitted here) rarely differed by more than .1%. It is a case therefore in which the increasing importance of the radiation forces renders further finessing with the fibre of doubtful use in the given environment. Logarithmic Decrements X log e.-In the endeavor to cope with this formidable difficulty, I began a study of the vibration of the needle in high vacua of a few thousandths of a millimeter; but the summer vanished before I completed it. The results so far obtained are interesting, however, and are given in the attached chart. The case was exhausted in the beginning to about 2.6 X 10-3 mm. on the McLeod gauge, and then sealed off. Air, however, in the lapse of 11 days very slowly leaked in through the glass cock and rubber tubes; and as the installment prevented me from finding the successive vacua (always beyond the U-gauge limit) I have expressed 64 PH YSICS: C. BAR US PROC. N. A. S. the results for T and X in the lapse of time. To determine the latter, the needle was deflected from rest to the maximum elongation, by placing the lead balls M M' in either of the active positions; and with the needle so deflected for an instant, to quickly put the weights in the neutral position again; i.e., in the vertical plane through the needle. It is thus possible to determine both the period T of the needle and the logarithmic decrement: the former by timing successive passages (first and second) through the equilibrium position with a stopwatch, and the latter by reading the elongations. The zero point was nearly constant. The radiation is thus taken symmetrically from both sides. So far as the method goes, the values of T and X are quite accurately found in this way, the difficulty being in the interpretation. The plenum value of X log e, so far as determinable in a heated room was about 1, the needle being now practically aperiodic. This has decreased to about .27 (ratio of arcs 1.9) at 2.6 X 1Omm. of pressure and the curve suggests a more r