. The Bell System technical journal . cy to rattleintroduced by unevenness in the clamping surfaces. Another feature in connection with the sound box is the needle-arm bearing shown in Figs. 17 and 18. Ordinary knife-edge bearingsare not sufficiently rigid as fulcrums and the rotational reactance aswell as the rotational resistance is undesirably large. A constructionwhich has been found to meet the necessary requirements is the ballbearing type with the steel balls held in position by magnetic making the ball-containing case of soft steel and magnetizing theshaft, it has been possible
. The Bell System technical journal . cy to rattleintroduced by unevenness in the clamping surfaces. Another feature in connection with the sound box is the needle-arm bearing shown in Figs. 17 and 18. Ordinary knife-edge bearingsare not sufficiently rigid as fulcrums and the rotational reactance aswell as the rotational resistance is undesirably large. A constructionwhich has been found to meet the necessary requirements is the ballbearing type with the steel balls held in position by magnetic making the ball-containing case of soft steel and magnetizing theshaft, it has been possible to manufacture this bearing reliably andcheaply. RECORDIXC, AM) RUJKOnUCl\C. 01 MUSIC JXl) SlLLCll 521 The horn which has been used as a terminating resistance to themechanical filter structure is a logarithmic one. The general propertiesof logarithmic horns have been understood for some time.^ There are two fundamental constants of such a horn—the first is thearea of the large end and the second the rate of taper. The area of the. Fig. 18—Sectional drawing showing construction of the system shown in Fig. 17 mouth determines the lowest frequency which is radiated energy of the frequencies below this is largely reflected if it is per-mitted to reach the mouth. From the equations given by Webster, ^^ it can be shown that alllogarithmic horns have a low frequency cut-ofT which is determined bythe rate of taper. If the rate of taper is so proportioned that its result-ing cut-off prevents the lower frequencies from reaching the hornmouth, the horn will then radiate all frequencies reaching its mouthand very little reflection will result.^^ It is, therefore, possible to builda horn having no marked fimdamental resonance. Webster, A. G., Acoustical Impedance and Theory of Horns and Phonograph,Proc. Nat. Acad, of Sci., 1919. ^ The authors wish to express their appreciation in this connection of the work ofMr. P. B. Flanders who carried out the mathematical inves
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