Certain Detectors used in Wireless Telegraphy. 537 



when the zincite was maintained at a potential about 0*45 

 volt below that of the pyrite. The power curves are given 

 in fig. 8 ; here a is the curve when no electromotive force 

 was applied : h is the curve for an applied electromotive 

 force of 0*45 volt, zincite negative, and c is the curve for 

 an applied electromotive force of 0'45 volt, zincite positive. 

 It will be seen from these that the combination forms a very 

 sensitive detect*. r, even when no external electromotive force 

 is applied. The curves showing the change in sensitiveness 

 with variation of the applied electromotive force are plotted 

 in fig. 9. 



Graphite- Galena. — A detector that is very widely used for 

 every-day telegraphy is that consisting of a pointed piece of 

 graphite touching the face of a crystal of galena. The curves 

 connecting applied electromotive force and current flowing 

 through the contact appear in fig. 10. The power curves 

 are shown in fig. 11 : curve a is obtained when the external 

 electromotive force is not applied, and curves b and c when 

 electromotive forces of 0*45 volt and —0*45 volt were applied. 

 The connexion between the power given to the telephone 

 and the electromotive force applied to the detector is given 

 in fig. 12. 



These two last defectors and others similar to them are 

 sometimes called " rectifiers," sometimes "thermoelectric 

 detectors." They are styled thermoelectric because it was 

 originally supposed that they owed their power of detecting 

 high frequency vibrations to the thermoelectremotive forces 

 set up at the contact by the rise of temperature produced at 

 that point — the point of highest resistance in the whole 

 oscillation circuit — in obedience to Joule's law ; but most 

 observers have concluded from experiments with both direct 

 and alternating currents, that these detectors derive their 

 function from an unexplained and hitherto unknown power 

 of rectifying rather than from a combination of the Joule 

 and Peltier effects. Pierce* has examined the behaviour of 

 contacts made with anastase, brookite, and molybdenite under 

 alternating currents of ordinary frequency, and obtained 

 oscillograms of the current through them. No evidence of 

 thermoelectric or other integrative action was perceived in 

 the photographs. Austin f has examined quantitatively, also 

 by aid of slow alternating currents, the properties of detectors 

 consisting of contacts of silicon and steel, carbon and steel, 

 tellurium and aluminium. Brandesj and Raetenkrantz § 



* Phvs. Review, p. 153 (1909). 



f Bulletin, Bureau of Standards, v. p. 133 (1908). 



X EUMro. Zeitscfa: xxvii. p. 1015 (1900). 



§ Phys. Zeitsckr. ix. p. 911 (1908). 



