THE AID OF THE ACHROMATIC FRINGES. 65 



telephone depending adjustably from the sleeve a may be slid right and left 

 or rotated into any horizontal position relative to E, and the current obtained 

 measured by the vibrator. 



Instead of sliding the telephone it is sometimes convenient to rotate it 

 into various positions. One may observe that the cases b, a, c, figure 70, are 

 the more interesting, since the current alternations of b and c are necessarily 

 opposite in phase, whereas e and /, h and * are not so. Hence the first design 

 b, a, c, is to be chosen. 



In the endeavor to minimize the mechanical coupling, the vibration tele- 

 scope was first put on a massive standard and additionally weighted. This, 

 however, had no advantageous effect of consequence. The telescope (separ- 

 ately mounted) was then placed at about a meter from the vibrator. In such 

 a case the phase difference of the vibrations of fringes and objective was 

 annulled, but the bands in the absence of current were nevertheless somewhat 

 oblique to the direction of the vibration of the objective, showing that the 

 fringes still vibrated. 



With this exception (presently to be removed) the behavior of the telephone 

 inductor was admirable. In passing from the positions b to c by sliding the 

 telephone, the ellipses regularly passed through the oblique bands, indicating 

 that these successive ellipses, even if of nearly equal size, were opposite in 

 their phase rotation. This was the case when the secondary was closed with 

 5,000 ohms and the inevitable inductance; also when a capacity was placed 

 in the secondary, and finally on passing from an inductance to a capacity in 

 the secondary. The effect produced by changes of capacity of 0.5 microfarad 

 was marked. The alternate-current effect, moreover, was still apparent when 

 the circuit was closed with 25,000 ohms and the telephone practically silent. 



There are, however, some peculiarities of behavior when a capacity is 

 inserted which need elucidation. The telephone responds strongly for capaci- 

 ties exceeding 0.05 microfarad, but the vibrator is often less influenced than 

 when the same loudness of telephone is produced by closing the circuit with 

 a resistance. Moreover, large ellipses are more apt to pass through bands 

 than small ones, which may be interpreted as an oscillation, or as the mere 

 dying away of strong antecedent vibratory motion and does not necessarily 

 involve changes of phase in the final result. 



The most immediate criterion as to changes of phase is the rotation of 

 ellipses as indicated in figure 72. If this rotation is in appearance counter- 

 clockwise when the telephone passes from positions b to c, figure 70, it will 

 be clockwise on the return passage from c to b, remembering that marked 

 changes of amplitude are involved and that the changes are gradual in corre- 

 spondence with the mass of the vibrator. Nevertheless, figure 72 and its sug- 

 gestion of the rolling of ellipses into place is not adequate, as it leaves out the 

 usual oscillation of ellipses before they become stationary. One must train 

 oneself to follow the sliding of any one of the four points of contact, within 

 the given vibration rectangle. I shall give a few examples of what is observed 

 in the telephone displacement in question. 

 5 



