130 Dr. W. H. Eccles on an Oscillation Detector 



current. This theoretical conclusion agreed well with the 

 experiments on an iron oxide coherer. The object of the 

 present paper is to show that the above hypothesis holds 

 good for a very different type of detector. 



All the well-known forms of " contact " detector consist of 

 a contact between two substances that stand well apart in 

 the thermoelectric series ; and the thermoelectric force plays 

 a very important part in their operation. They are usually 

 classed as " rectifying detectors " to distinguish them from 

 such detectors as the coherers. In seeking a " rectify in cr 

 detector " that would illustrate the above hypothesis, sub- 

 stances that stand far apart in the thermoelectric series 

 must be avoided, for such substances would introduce Peltier 

 effects that would tend to mask the resistance-temperature 

 phenomena we wish to isolate. Even in the iron : iron- 

 oxide : iron coherer it is possible that thermoelectric forces 

 arise owing to unequal heating of the two iron to iron-oxide 

 junctions. It occurred to me, therefore, to construct a 

 detector out of one substance only. Accordingly a search 

 was made for a substance possessing high, but not too high, 

 resistivity, with large negative temperature-coefficient. 

 Several such substances were found among the native crys- 

 talline oxides and sulphides. Most of these are a?lotropic, 

 and must on that account be avoided. Fortunately galena, 

 the native sulphide of lead, has fairly high resistivity, a very 

 large negative resistance coefficient, and crystallizes in the 

 cubic system. A galena-galena detector was therefore 

 constructed. Two pieces of galena cut from the same 

 crystal were embedded in solder, mounted in a clamp, and 

 brought into gentle contact. It was put into a wireless 

 telegraph receiving set and was found to yield excellent 

 signals when a current of proper magnitude was passed 

 through it. 



A few preliminary experiments showed that it was not 

 easy to find two pieces of galena, which when placed together 

 and submitted to positive and negative voltages in turn, 

 exhibited perfect symmetry. The asymmetry is always 

 small, and is probably due to the rise of thermoelectric forces 

 at the contact when it becomes heated by the steady current, 

 and these forces are probably caused by slight variations in 

 the chemical composition of the galena from point to point 

 in the crystal. When a contact that gives symmetrical 

 voltage-current curves is found, it constitutes a detector that 

 operates only when a current is passing through it. The 

 efficiency of the detector is the same for each direction of the 



