RECTIFYING PROPERTY IN SILICON AND SELENIUM. 133 



side of it. This is shown to a more marked extent by 

 Curve III, which gives the apparent conductances (that is, 

 the ratio of current to e.m.f.) as ordinates, the abscissae 

 representing values of the e.m.f. as in the other curves. 

 In Curve II the unit of e.m.f. is T 4 T volt and the unit of 

 current T ±o ampere ; for higher values than those shown 

 the rectification becomes greater. 



Several curves similar to those shown were obtained, 

 the form being the same under various conditions and with 

 various metals making contact with the silicon ; steel, 

 brass, copper and platinum were used for one or both sur- 

 faces in different trials. The position of the minimum 

 varied on different occasions, but the e.m.f. at the point of 

 minimum conductance was always less than one volt. 



With the object of obtaining further information about 

 the minimum conductance, some trials were made in which 

 two steel needles or two copper wires with rounded ends 

 were pressed against the piece of silicon, held as described 

 above, at points about one millimetre from the brass plates. 

 The difference of potential between any two of the four 

 metal contact pieces, namely the two brass plates and the 

 two wires or needles, could be determined by means of a 

 potentiometer and the value of the current measured by 

 observing the difference of potential between the ends of a 

 standard resistance coil joined in series with the silicon. 



It was found that the difference of potential between the 

 brass plate at the apex of the pyramid and the contact wire 

 one millimetre from it was always much greater than the 

 difference at the base, and that between the two contact 

 wires was but a small fraction of either, so that the actual 

 resistance of the silicon was very small compared with 

 the contact resistances. 



Both at the base and apex it was found that, when the 

 current flowed from the brass to the silicon, the difference 



