OF ARTS AND SCIENCES. 93 



which increase is succeeded by a diminution of current when the mass 

 is still further increased. This will be apparent from an inspection of 

 the curve shown in Figure 2, which illustrates in a general manner 

 the relation of the pressure between the electrodes to the current in 

 the primary circuit. 



In this the momentary changes in pressure, Ap, Ap', Ap", etc , due 

 to the sound-waves, and corresponding to loads and normal pressures 

 p, p\ p", etc., have increasing values within certain working limits, 

 owin£f to the increasing mass of the anvil electrode. The currents in 

 the primary also increase, though at a gradually diminishing rate, as 

 the pressure between the electrodes is increased, so that the increments 

 of current, Ac, Ac', Ac", corresponding to the pressure-changes Ap, 

 Ap', Ap", have increasing values up to some point, as a, after which 

 they decrease. This being the case, it is evident that the current in 

 the secondary will at first increase to a maximum, and afterwards 

 diminish, since the currents in the secondary corresponding to press- 

 ures p, p', p", etc. will be proportioned to Ac, Ac', Ac", etc., and this 

 is precisely the curve which is obtained in the experiments. The 

 explanation just offered seems therefore to be the' true one. 



The matter was still further tested by carrying out a set of experi- 

 ments similar to those already described, except that the variations in 

 normal pressure were brought about by means of a spring instead of 

 by adding weights. In such a case the successive values of Ap in 

 the curve (Fig. 2) would be of the same magnitude, while Ac would 

 continually diminish. The current in the secondary should therefore 

 have its maximum value when the initial normal pressure is least, 

 and continually diminish as that pressure becomes greater. 



The experimental results verified this conclusion, as will be seen by 

 reference to Figure 3, in which the normal pressures are represented 

 by abscissas, and the currents in the secondary by ordinates, as before. 

 The curve is approximately a straight line. It is possible that the 

 deviations from this are due to instrumental imperfections, as the 

 apparatus used did not allow of more than an approximate determi- 

 nation of the pressure applied by the spring. 



Eogers Laboratory of Physics, 



June, 1888. 



