May 27, 1921] 



SCIENCE 



491 



found to increase from zero, with great 

 rapidity to a iwsitive maximum. The de- 

 flection then falls off with similar rapidity 

 through zero to the negative value when the 

 cock is again quite closed. I have indicated 

 this result graphically in Fig. 2, in which the 

 abscissas show the d^ree to which the cock 

 has been opened and the ordiaates the fringe 

 deflections, s, obtaiaed. The maximum pres- 

 sure obtained in these initial experiments was 

 the equivalent of about 50 fringes; i.e., 

 Ap = 1.5 X 10-= cm. or about 2 X 10"= atmos- 

 phere for a frequency of about 12 per second. 

 At higher frequencies this datum is much 

 increased. 



These pressures are real: for on suddenly 

 closing the cock at the maximum and break- 

 ing the current, they are retained until dis- 

 charged on opening the cock. 



4. Pressure Depending on the Frequency 

 and on the Intensity of Tihration. — The 

 maxima are observable for very considerable 

 reductions of the intensity of vibration. In 

 Fig. 3 curves 3, 6, I have given examples of 

 the observed fringe displacement, s, when 

 different resistances (given by the abscissas in 

 10^ ohms) are put in the telephone circuit. 

 In curve 3 the frequency is n. = 12 per second. 

 Curve 5 contains similar results when the 

 frequency is «. ^ 100 per second. The sensi- 

 tiveness has obviously greatly increased and 

 in a general way this is the case for higher 

 frequencies. 



5. Fringe Deflection Varies as Current In- 

 tensity. — The graphs. Fig. 3, are roughly 



■hyperbolic, so that the equation rs= C (r be- 

 ing the high resistance inserted into the tele- 

 phone circuit) may be taken to apply within 

 the errors of observation for resistance ex- 

 ceeding 1,000 ohms. So computed for con- 

 venience rs is 24 X 10= i^i series 3 and 

 36X10= in series 5. Hence at r=100 ohms 

 the pressure would have been Y X 10"= and 

 1.1 X 10"^ CTn. of mercury. The instrument 

 taken as a dynamometer is thus noteworthy, 

 since its deflections would vary as the first 

 power of the effective current or i ^ i^s. It is 

 of interest, therefore, to ascertain how far the 

 sensitiveness which can not here be estimated 



as above 10-* amperes per fringe, may be 

 increased. 



6. Pin Hole Sound LeaTcs. — Pin holes less 

 than a mm. in diameter seem more like a pro- 

 vision for light waves, than for sound waves 

 often several feet long; but one may recall 

 the phenomenon of sensitive flames. 



It is so difficult to make the fine adjust- 

 ment for maximum conditions with stopcocks 

 that their replacement by the devices given in 

 c and c. Fig. 1, is far preferable. Here c is 

 a quill tube, to one end of which a small 

 sheet of very thin copper foil has been fast- 

 ened with cement The sound leak at is 

 then punctured with the finest cambric needle. 

 The other end (somewhat reduced) is thrust 

 into a connector of rubber tubing at t". In 

 case of c' the tube has been drawn out to a 

 very fine point. This is then broken or 

 ground off until the critical diameter (.04 cm.) 

 is reached. Both methods worked about 

 equally well but in the case c several holes 

 side by side or holes of different sizes may be 

 tried out. Such results are shown in Fig 4, 

 which exhibits the deflection (s fringes, ordi- 

 nates) for different diameters of hole in mm. 

 (abscissas), when 1,000 ohms were put in the 

 telephone circuit. It will be seen that the 

 optimum .4 mm. in diameter is quite sharp. 

 The finest size of needle is needed. 



An example of results obtained with the 

 sound leak c when different resistances are in 

 circuit, is given in Fig. 3, curve 8. The value 

 of rs; viz., 



* 51 25 16 12 10 5 fringes 



10-=r 1 2 3 4 5 10 ohms 



lO-'r* 51 50 48 48 50 50 



is much more constant than hitherto and 

 reaches 50 X 10"=. Hence at 100 ohms the 

 pressure increment should be Ap = 1.5 X 10'^ 

 cm. of mercury. 



Figure 5 finally indicates that the multi- 

 plication of pinholes, all of the same diameter 

 (.04 cm.) is similarly disadvantageous. The 

 deflection for four holes is scarcely half as 

 large as for one. 



If with the current on, a drop of water is 

 placed on the hole in c. Fig. 1, the pressure 



