ACOUSTICS AND GRAVITATION. 23 



astonishingly large pressure decrement ( Ap = 0.04 cm. of mercury at 100 ohms) 

 now lurked in this region. Clearly, therefore, the pressure decrements depend 

 on something more than the mere instrumental error suspected, and that 

 their occurrence must be as pronounced as the pressure increments which 

 they reciprocally precede or follow, the next paragraph will show. 



A final experiment may here be inserted. Using the bipolar telephone above 

 at a frequency of 100 and with 1,000 ohms in circuit, the usual pin-hole leak 

 c was replaced by a fine screw stop-cock (at C, fig. 1 4) . Here the degree of 

 opening could be specified in degrees of turn of the screw. The results obtained 

 were (after the lapse of about a minute) : 



Cock open, o 45 90 180 quite open 



Fringe deflection, s = 34 20 12 2 +2 fringes 



The pressure production at a second very small leak thus very soon counter- 

 balances the gradual dissipation of air at the clamped telephone plate. 



22. Reversal of poles of telephones changes sign of fringe deflection. An 



earlier detection of this result would have saved me much mystification. Not 

 expecting it, I did not look for it, but it seems that a reversal of the telephone 

 current (so to speak) frequently reverses the fringe deflection symmetrically. 

 Thus when a switch (positions 7, //) was added to the telephone the fringe 

 deflections were 



r 5j s z n 



2,000 ohms +27 26 i oo per second 



15 16 12 



1,000 48 47 100 



49 -47 100 



This is as close to an exact reversal as the quivering fringes can guarantee. 

 To test the case further, I used the motor interrupter, making a survey for 

 frequencies between g' and a", as shown in figure 30. The motor did not run 

 very smoothly, so that the estimation of pitch was difficult; but the curve, 

 figure 30, corresponds to the curve, figure 28, except that maxima and minima 

 have been reversed by the switch. The maxima now dip into the positive 

 region. Thus the apparatus, regarded as a dynamometer and with precautions 

 as to frequency, would give both quantity and sign of the effective currents 

 in the telephone. Moreover, in any given adjustment, pressure increments 

 changing continuously into decrements, and vice versa, are not unusual. 



Since the resonating region R is vented by the pin-hole, the positions of 

 equilibrium of the quiet and vibrating plate are ineffective. Hence it is neces- 

 sary to assume that the vibrations of the plate are not symmetrical; or that, 

 for instance, the impulse corresponding to the break of current at the inter- 

 rupter is of excessive importance. Thus, if this current re-enforces the 

 permanent magnetization, the effect is a dilatation; if it interferes with the 

 magnetization, the effect is a pressure. 



If the plate is energized symmetrically (as, for instance, by a small magneto 



