ACOUSTICS AND GRAVITATION. 



17 



and 36Xio 3 in series 5. Hence, at r=ioo ohms (taken as a standard), the 

 pressure would have been yXio~ 3 and i.iXio~ 2 cm. of mercury. The in- 

 strument considered 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 esti- 

 mated as above io~ 4 amperes per fringe, may be increased. 



18. Pin-hole sound-leaks. Pin-holes less than a millimeter in diameter 

 seem more like a provision 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 adjustment for maximum conditions with 

 stop-cocks that their replacement by the devices given in c and c', figure 14, 

 is far preferable. Here c is a quill-tube, to one end of which a small sheet of 

 very thin copper foil has been fastened 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 (0.04 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 figure 19, 

 which exhibits the deflection of fringes (ordinates s) for different diameters 

 of hole in millimeters (abscissas), when 1,000 ohms were put in the telephone 

 circuit. It will be seen that here the optimum 0.4 mm. in diameter is quite 

 sharp. The finest size of needle is needed. 



The results obtained with the sound-leak c (when different resistances r 

 are in circuit) are given in figure 20 for three diameters. As a whole the sensi- 

 tiveness has been much increased, which is additionally shown by the com- 

 parison in figure 18, series 8. The value of rs, viz, 



5 = 51 25 1 6 12 10 5 fringes 

 io~ 3 r= i 23 4 5 10 ohms 

 io~ 3 rs = $i 50 48 48 50 50 



is also much more constant than hitherto and reaches 50 X 1 ^ 3 - Hence at 

 100 ohms the pressure increment should be Ap 1.5 X io~ 2 cm. of mercury. 



Figure 21, finally, indicates that the multiplication of pin-holes, all of the 

 same diameter (0.04 cm.), is similarly disadvantageous. The deflection for 

 four holes is scarcely half as large as for one. 



