224 O. U. VONWILLER. 



amperes being obtained by multiplying tlie numbers given 

 by 0*000266. The ordinates are proportional to the differ- 

 ences of potential between the pairs of contacts, the + sign 

 indicating that the current flows from A to D while the - 

 sign indicates a flow in the opposite direction; thus the 

 curve +0 D gives the relation between the potential differ- 

 ence between O and D and the current when the direction 

 of flow is from A to D. In the case of this specimen the 

 drop of potential between A-B was much greater than 

 those between B - O and C-D, and the scale of ordinates 

 is five times as great for curves A D and A B as for C D 

 and B O. The potential differences in volts are obtained 

 by multiplying the numbers given by 0*00133. Three 

 different zeros have been taken for the ordinates in order 

 to prevent confusion. 



Several points may be noted in connection witli these 

 curves : — 



(1) The resistance of the whole, proportional to the ratio 

 of ordinate to abscissa for the curve A D, is not constant, 

 varying with the magnitude and direction of the current. 



(2) At each of the end pairs of contacts A B and O D 

 the potential difference for a given current is greater when 

 the current enters than when it leaves. This was found 

 to be the case with all specimens tested. 



(3) For the higher values of current the curves +A D, 

 — AD, +AB and — A B are convex upwards, showing 

 resistance decreasing as current rises. Readings for cur- 

 rents considerably greater than those shown in figure 2 

 give a like effect for the curves +OD and -OB. 



(4) For very low currents the curve +A B is concave 

 upwards, there being a change of curvature later, that is 

 the effective resistance between these points increases at 

 first as the current rises from zero, decreasing later. Read- 

 ings with larger currents show alike effect with -CD. 



