490 



SCIENCE 



[N. S. Vol. LIII. No. 1378 



other break. Large resistances are to be put 

 in the telephone circuit so that the induct- 

 ances are of secondary importance. The bore 

 of the tubes t, t' , c, c' need not exceed 5 mm. 

 Thus the chamber in B, about 6 cm. in 

 diameter and 2 cm. deep, is the resonator 

 (capacity 57 cm.^) of the apparatus. 



When the cock C is closed there is no 

 appreciable effect until the telephone resounds 

 harshly. In such a case there is marked 

 dilatation in the resonator R, increasing with 

 the intensity of vibration. The successive 

 readings (s' fringes) are liable to be fluctu- 

 ating, but the sign and mean value is definita 



f ^' a' c d' e o' a" ■ d" t" a' a" 



Tigs. 1 to 8 



The displacements of the achromatic fringes 

 corresponding to the head of mercury in B 

 may be read off by a telescope provided with 

 an ocular .1 mm. micrometer. It is perhaps 

 advantageous to place the micrometer in the 

 wide slit of the collimator, the frioges being 

 parallel to the scale parts. To obviate the 

 need of adjusting the inclination of the 

 fringes (as this frequently changes), the slit 

 holder should be revolvable around the axis 

 of the collimator, the scale being parallel to 

 the length of the slit and the fringes moving 

 in the same direction across the white ribbon- 

 like field. Fringes equal to a scale part in 

 breadth are most convenient. 



2. Ohservations. Closed and Open Resona- 

 tors. — Spring interrupters dipping in mercury 

 were first used, having frequencies of n ^ 12 

 and 100 i)er second, respectively. 



Since for s' — s^^s the head is sX/2 (the 

 displacement being s fringes of wave-length 

 A.), this mean value, s^7 fringes for the 

 given intensity of vibration, is at once equiva- 

 lent to Ap = 2 X 10"* cm. of mercury, or to 

 about 3 X 10"^ atmosphere. If but 500 ohms 

 are put into the telephone circuit, however, 

 appreciable deflection ceases. 



Again, if the stopcock is completely open 

 no effect whatever is obtained. The bore of 

 the small stopcock in this case need not ex- 

 ceed 2 or 3 mm. All the negative results 

 which I obtained by other methods heretofore 

 are thus explained. 



3. Resonator All hut Closed. — If now the 

 plug of the cock C is rotated from the open 

 position gradually until the opening is re- 

 duced to the merest crevice, the fringe de- 

 flection s will, on further slow rotation, be 



