CHAPTER X. 



ATMOSPHERIC ELECTRICITY. 



Owing to the large amount of work entailed by the ordinary meteorological and magnetic 

 investigations undertaken by the expedition, it was found impossible to do more in atmospheric 

 electricity than to measure the potential gradient and the radio-active contents of the atmos- 

 phere. Both of these unfortunately came to an end after the first year's work, as the few 

 men who remained in the south during the second year had their hands full with other 

 duties. 



Potential gradient. 



Only thos3 who have attempted maasurements of atmospheric electricity under Polar condi- 

 tions can have any idea of the great difficulty of the work and the constant attention which 

 the apparatus requires. Th> difficulties met with in the ordinary meteorological observations 

 have not been described in the above discussion as they can be overcome with a little fore- 

 thought, and each observer must work out the bast means to meet his own difficulties, 

 but with atmospheric electricity so few observers have had the necessary experience that a 

 few words here of the difficulties met with and the way they were overcome may not be 

 out of place. 



The method of recording the potential gradient is well known. We have first the collector 

 which is exposed in a suitable position in the open air. The function of the collector is 

 to bring itself and the apparatus attached to it to the potential of the air in its immediate 

 neighbourhood. Until the discovery of the radio-active substances only two practical collectors 

 were known, (a) the Kelvin ' water dropper ' and (b) the flame. The first of these is quite 

 unusable in Polar regions owing to freezing, and the difficulties connected with the second 

 make its continuous use with self-recording instruments practically impossible. The radio- 

 active collector does not freeze and does not blow out, but in its usual form it can only 

 dispose of such a small amount of electricity that the question of insulation becomes 

 very important. Unless the insulation is practically perfect, the collector cannot dispose 

 of the electricity which leaks to the insulated system rapidly enough, and the apparatus 

 charges up to a potential more or less below the potential of the air near to the 

 collector and the apparatus then records a mixture of changes of potential and changes of 

 insulation. 



The collector used was a small copper rod about 3 mm. in diameter and 5 cm. long. 

 This had been coated with polonium which has the great advantage of emitting only a 

 rays, so that only the air in the immediate neighbourhood of the collector is ionised. The 

 collector was mounted on a thin wooden rod which in turn was fixed to an iron pole, so 

 that the collector itself was about ten feet above the ground. The wooden rod was insulated 

 from the iron pole by means of a plug of ebonite covered in suliihur. An inverted tin 

 can was placed over the insulator in the method shown in figure 89. This was found neces- 

 sary in order to protect the insulator from hoar frost which in certain conditions of the 

 atmosphere was found to be deposited on every thing exposed to the sky. 



