Physics. — ''The Unidirectional Resistance of Crystal Detectors''. 

 Bj M. J. HuTZiNGA. (Communicated bv Prof. H. Haga). 



(Communicated in the meeting of November 30, 19l8>. 



In a former communication (c.f. these Pioceedings September- 

 meeting 1916) the electrolj-tic phenomena of the molybdenite-detector 

 have been described. These phenomena made it likely that the 

 rectifying- power in this ci-ystal contact is not due to thermo-electric 

 effects as is usually thought to be the case, but to the E. M. F. of 

 electrolytic polarisation. 



The question whether one is entitled to extend this conclusion 

 also to other crystal detectors was discussed in my doctor-thesis 

 (Groningen 5 Juli 1918). 



The [)resent communication gives the results there described. 



^ 1. Nature of the electrolytic products in the wolyhdenite-contact. 



If a platinum point is placed on a molybdenite-crjstal and a current 

 of about one niilliampère is sent from the crystal to the platinum, 

 a small quantity of a dark blue liquid will be developed. The spot 

 of liquid will not expand with a stronger current, as the additional 

 heat of the current brings about a quicker evaporation. Therefore 

 repeatedly a tiny drop of distilled water was ptit on the place of 

 contact and every time was sucked up again as soon as, usually 

 after some seconds, it had got a dark blue colour. In this way a 

 few c.c. of this solution were obtained. In the chemical Laboratory 

 of Groningen (director Prof Jakgkk) this liquid was examined, leaving 

 after evaporation a l)lue residue MoO,, MoO», 6H,0 ; a colloidal sub- 

 stance which when exposed to the air, oxidised slowly into MoOj. 



If the current is sent through the detector in the opposite direc- 

 tion, e.g. from the platinum to the molybdenite, the brown substance 

 developed can easily be obtained in large quantities by electrolysis 

 of some diluted acid between electrodes of molybdenite. This brown 

 colour must be attributed to the colloidal sulphides MoS,, MoS,. 



§ 2. Other detectors. 



A second detector-combination in which phenomena of electrolysis 



