240 Mr. A. T. Waterman on the Positive 



The portion (a) of the curve represents the initial heating 

 before the break ; the group of points (/3) was taken after 

 the break ; the group (7) was obtained after the strip had 

 been heated for a long time in the second state. Lines are 

 drawn connecting the definite consecutive readings. The 

 dotted line represents the break or change in state. The- 

 portion (a) of the graph for low values of the potential has 

 the form of approaching the vertical axis asymptotically ; 

 the value of the resistance for a potential difference of two 

 volts was 40,100 ohms, for instance. Results of the same 

 general character as those indicated were obtained with 

 different strips, and when the resistances and potential 

 differences were measured in a variety of ways. 



As the break was approached the values of the resistance 

 for a given potential were observed to decrease with time ; 

 so that often a considerable interval elapsed before the 

 resistance reached a steady value. Beyond the break it was 

 often difficult to maintain a steady resistance. The resistance 

 of one of the strips under constant voltage was measured at 

 24° C, 100° C, 262° C, and 316° 0. The break was found 

 to occur at 310° C. Below the break the resistance varied 

 with the absolute temperature T in accordance with the 

 formula Ae b/T , where A and b are constants. Normally 

 molybdenum sulphide belongs to the hexagonal class of 

 crystals, type dihexagonal equatorial. In the experiments 

 the heating current was passed through the strips at right 

 angles to the crystallographic " c" axis. There is no evidence 

 of any change of crystalline or chemical structure at the 

 break unless a sudden rise in temperature observed at that 

 point can be so interpreted. The variation of resistance with 

 current for very small currents and the tendency to persist 

 in the second state, already alluded to, suggest that the 

 electrons are liberated in the material, partly by the action 

 of the electric field and not merely as a result of the increase 

 of temperature. Apparently the best condition for a quick 

 return from the second to the first state is the presence of a 

 small current, but one which is not zero. Professor A. 

 Trowbridge has shown that the sign of the Hall effect in 

 molybdenite indicates that the current is carried by negative 

 electrons. 



8. Emission. 



The first experiments to determine e/m, made by the 

 method used in dealing with the salts as described in the 



