134 On an Oscillation Detector, 



contact resistance and therefore a different curve. Since 

 very slight accidental vibrations can cause relative motion 

 of the pieces of galena, it is almost impossible to perforin a 

 series of measurements yielding sufficient data for all the 

 curves of figs. 1, 2, 3, with full confidence that the contact 

 has not varied. 



Curve a of fig. 1 follows the equation. 



=(n 



+ 4-0xl0 3 V - • (3) 



2-95 x 10V 



with fair accuracy ; curve h obtained with another setting 

 of the crystals has the equation 



-f2-5xl0 3 )e. ... (4) 



_/ 4M 



u+8-; 



) x 10 8 c 2 



The curve of fig. 2 was obtained with the same setting of 

 the crystal that gave curve a of fig. 1. It was shown in the 

 previous paper that the ordinates of this curve should be 

 proportional to m the coefficient of W in equation (2) above, 

 and that m contained the gradient of the steady current 

 curve as its principal factor. A comparison of figs. 1 and 2 

 confirm the deduction. The line of fig. 3 has the equation 



ic = 0'017 (W-0*4xl0 8 ), 



where W is the power in watts given to the detector in the 

 form of oscillations, and w the power transformed by the 

 detector and passed to the telephone circuit. 



The galena detector here investigated proves very good 

 in practical wireless telegraphy. It has shown itself to be 

 better than any coherer * known to me. This superiority is, 

 according to the hypothesis here advocated, to be ascribed 

 chiefly to the large negative temperature-coefficient of re- 

 sistivity of galena. By direct measurement of a cube of 

 galena clamped between pieces of tinfoil the resistance was 

 found to fall from 0*33 ohm at 12° C. to 0*10 ohm at 99° C. 

 — a negative coefficient of 0*0079 per degree centigrade. 

 Pyrites has a coefficient about 0*006. Iron oxide has a 

 coefficient somewhat lower than this last. 



The cost of a portion of the apparatus used in these 

 experiments was defrayed out of a grant from funds at the 

 disposal of the Royal Society. 



* But not so o-ood as certain " thermoelectric " detectors. 



