NEGATIVE ELECTRICITY FROM HOT CALCIUM AND FROM LIME. 



155 



Professor WILSON gives for the leak at 1545 C. the value 6'38 x 10~ 7 ampere. The 



smallest value I ever obtained for the negative leak in helium was 6'91 x 10~* ampere 



per square centimetre of platinum surface at 



1 540 C. By taking great precautions in cleaning 



his platinum wires and purifying the air in the 



apparatus, with a special view to getting rid of 



all traces of hydrogen evolved by the wire when 



heated, Professor WILSON was able to reduce 



the leak to 10~* ampere per square centimetre 



of surface at 1616 C. The values given in 



Table I. for the negative leaks in helium at 



different temperatures could be reduced to about 



one-tenth by reducing the pressure of the gas in 



the apparatus. Since Professor WILSON'S results 



were obtained in a good vacuum, it seems that 



the value of the negative leak in helium is 



practically the same as in air under similar con- 



ditions of temperature and pressure. 



From the numbers in Table I. the curve in fig. 3 was drawn. It is similar to the 

 current-temperature curves obtained by other observers in air. 



O. W. RICHARDSON and H. A. WILSON have found that the variation of the negative 

 leak with temperature can be expressed by an equation of the type 



tempera.tune centigrade 

 Fig. 3. Relation between negative leak 

 and temperature of platinum cathode in 

 helium gas at 3 236 millims. pressure. 



x = 



where x is the current in amperes, the absolute temperature, and Q and A are 

 constants. Q is a measure of the work done by a corpuscle in escaping from the 

 surface of the metal ; it can be obtained from any two values of x by means of the 

 equation 



0_o 



By the use of this formula the following values of Q were obtained from the observed 

 currents. The numbers in brackets refer to the observations in Table L, which were 

 used in calculating the value of Q. 



TABLE II. Values of the Constant Q calculated from Observations of Table I. 



From oltservations 



Mean temperature, 



1400 

 1505 

 1557 

 1591 



Q 



(calories). 



121,100 -| 

 lL':.,100 L, 

 121,800 f Mean 

 1 19,500 J 



121,900. 



X 2 



