due to Heating Aluminium Phosphate. 579 



reached its normal value. The magnitude of the increased 

 current was found to depend upon the time during which e 

 was insulated. It gradually increased with the time, and 

 readied a maximum when the insulation had lasted ten 

 minutes. Any longer period of insulation was found to have 

 no further increasing effect upon the current. 



It is possible that during the time of insulation positive 

 ions are being freed in the salt on l e' these accumulate in 

 the substance and are prevented from escaping by a discon- 

 tinuity of the potential at the surface. This continues until, 

 at the end of 10 minutes, the field due to the accumulated 

 charges is sufficient to take them over as they are formed, 

 when of course no further increase in the number accumu- 

 lated takes place. 



When the field is again put on these ions are of course 

 dragged out, and the initial value of the increased current 

 will depend upon the number of ions which have accumulated. 



This effect greatly increased the labour of taking readings 

 under varying conditions, as considerable time had to be 

 allowed after a change had been made for the current to 

 become steady. These experiments were carried out after 

 the steady state had been reached. 



This effect was more apparent in some gases than in others, 

 of those tried it was most marked in the case of C0 2 . 



V. Effect of Pressure upon Current. 



When the salt is in the steady state it is most convenient 

 for investigating the changes due to alterations of pressure 

 and temperature. 



The changes in the current when the temperature was kept 

 constant while the pressure was varied were first undertaken. 

 The contained gases used were air and C0 2 . The results ob- 

 tained were of a similar nature for both these gases. 



Some typical curves are shown in the accompanying 

 diagram (PI. XL fig. 4) in which the pressures are plotted as 

 abscissae, and the currents as ordinates. 



From these it can be seen that starting from zero pressure 

 there is a very rapid rise in the current in all cases, the 

 higher the temperature the more rapid being the rise. For 

 each temperature the current reaches a well-marked maximum 

 value. This, again, is more pronounced at the higher 

 temperatures. When the maximum current has been 

 obtained, any further increase in the pressure produces an 

 immediate and rapid decrease in the current. For the 

 temperatures investigated, this decrease continues until the 



2Q2 



