Conductivity of Air and Salt Vapours. 



229 



<2.) Variation of the current with the E.M.F. 

 (3.) Variation of the current through air with the temperature. 

 (4.) Variation of the current through salt vapours with the tem- 

 perature. 

 (5.) Summary of results. 



The relation between the current and E.M.F. in air was found to 

 •depend very much on the direction of the current. When the outer 

 •electrode was negative the current attained a saturation value with an 

 E.M.F. of about 200 volts, but when the outer tube was positive it 

 increased rapidly with the current, even with an E.M.F. of 800 volts, 

 so that a much greater E.M.F. would be necessary to produce satura- 

 tion, that is, assuming that saturation can be produced at all. 



With salt vapours the relation between the current and E.M.F. was 

 not much affected by reversing the current. The current was always 

 greater when the outer tube was negative, the reverse being the case 

 with air alone. At low temperatures the current attained a saturation 

 value, but above 1000° C. it was found to increase more nearly pro- 

 portionally to the E.M.F. 



The variation of the current at constant E.M.F. with the temperature 

 for air was found to be approximately capable of being represented by 

 a formula of the type C = A6 n , where G is the current, 6 the absolute 

 temperature, and A and n constants. The constant n depends on the 

 E.M.F. used. With 240 volts it was 17, and with 40 volts 13. The 

 current, therefore, does not begin suddenly when the temperature is 

 raised, but always increases regularly with the temperature, so that the 

 lowest temperature at which the current can be detected depends 

 entirely on the sensitiveness of the galvanometer. 



The energy required to ionize 1 gramme molecular weight of air was 

 estimated by supposing that the fraction of the gas dissociated into ions 

 is proportional to the current at small E.M.F's. By means of the 

 ordinary thermo-dynamical formula giving the variation of the dissocia- 

 tion with the temperature, the energy in question can then be obtained. 

 The result for air is 60,000 calories between 1000° and 1300° 0. This 

 amount of energy is of the same order of magnitude as the energy set 

 free when H and OH ions combine to form water in a solution. 



The relation between the current and temperature for salt vapours 

 was found to be rather complicated. With KI, using an E.M.F. of 

 300 volts, the current had the following values (1 = 10~ 4 ampere) : — 



Temperature 500° 600° 700° 800° 900° 1000° 



Current 0'7 1-8 3'0 4*0 4 -5 4-0 



Temperature 1100° 1150° 1200° 1300° 



Current 3*5 3*6 7'0 7-0 



