536 Dr. W. H. Eccles on the Energy Relations of 



electromotive amplitudes of 1/10000 volt, and that oscilla- 

 tions produced by spark-discharges in the laboratory affected 

 the detector equally whether the small electrode was anode 

 or cathode. He concluded that in the action of the instru- 

 ment heat had a share, and that chemical action, electrostatic 

 attraction across the gas film, and also a property styled 

 rectification, all took part. 



Carborundum. — The detector was set up by clamping a 

 crystal of carborundum between brass plates, so that a smooth 

 crystalline edge or corner was in contact with one plate, and 

 a blunt and more amorphous part of the crystal in contact 

 with the other plate. The steady current curves of two 

 crystals widely different in their electrical behaviour appear 

 in fig. 4 (PL X.). Curves a b belong to one crystal, the dotted 

 curves AB belong to another. The upper curve of each pair 

 was obtained when the jagged blunt end of the crystal was 

 positive. 



The power curves are given in fig. 5. Lines abc exhibit 

 the energy relations for the crystal that gave a b in fig. 4. 

 Line a was obtained while the blunt end was at a potential 

 2*62 volt higher than the smooth end ; b was got when the 

 potential difference was — 0'44 ; curve c was got without 

 electromotive force. Lines A, B, C refer to the other crystal. 

 Line A was obtained while the blunt end was 2*9 volt above the 

 smooth end ; line B while the blunt end was 2*1 volt below ; 

 line C while no external electromotive force was applied. 

 The curve of fig. 6 shows how sensitiveness altered with the 

 electromotive force applied to the terminals of the detector. 



The Carborundum detector has been examined very ex- 

 haustively by Pierce*. He has shown that crystals of this 

 substance may be as much as 1000 times more conductive 

 for current in one direction than in the opposite, and has 

 concluded that the substance acts as a detector of high 

 frequency oscillations solely because of this unilateral con- 

 ductivity. Heat, he considered, played no part in the process. 

 The curves given above show, however, that a crystal may 

 be a good detector even though its unilateral conductivity 

 be not very pronounced. 



Zincite-chalcopyrite. — The detector made by arranging a 

 corner of a fragment of brown zincite (native oxide of zinc) 

 to press against a piece of chalcopyrite (iron copper sulphide) 

 is one of the most sensitive known. It is used extensively 

 in various navies. Fig. 7 gives the results of measurements 

 of current under steady electromotive force. For this par- 

 ticular detector the most sensitive condition was attained 

 * Phys. Keview, xxv. p. 31 (1907). 



