520 



The scale values of this electrometer as determined by me after the return of the expedition were 

 found to differ considerably from the values obtained at Cape Town on the outward voyage. The 

 constant, by which the readings on the divided head of the instrument had to be multiplied to reduce 

 them to volts, originally varied from 1-6 when the vertical scale reading was to 2'0 when this reading 

 was 30; whereas after the return of the expedition the constant varied from 2'6 for a vertical scale 

 reading of 6 to 3'0 for a vertical scale reading of 20. This change of the constant of course introduces 

 a serious uncertainty into the results, as it is impossible to say definitely when it took place. The change 

 of constant is most probably due to injuries received on the outward or homeward voyage or during 

 loading or unloading operations. Whether, however, the change of constant took place before or after 

 the observations were taken it is impossible to determine. There is a record of an accident to the 

 electrometer a month or two after observations at the Winter Quarters were begun. After the 5 p.m. 

 observation on April 24, 1902, there is the following note: "The observer (L. B.), immediately after 

 taking the above observation, accidentally tilted the instrument while putting it back in its box, which 

 loosened and shifted the guard plate and discharged the instrument, and necessitated its being taken to 

 a warm room to be overhauled. The guard plate was gently pressed back into its place and the instru- 

 ment thoroughly dried." It is just possible that the change of constant may have occurred at this time. 

 No further accident appears to have happened to the electrometer while the observations continued. It 

 is unlikely, therefore, that the relative values of potential recorded subsequently to this date were 

 affected. 



In the reduction of the results the scale values determined after the return of the expedition have been 

 used, but the uncertainty resulting from the change in the constant of the electrometer must be borne 

 in mind. 



As " collectors " the ordinary electrometer match or fuze was used. The observations, except where 

 otherwise described, were taken with the electrometer placed on a tripod stand, the table of which was 

 about 4 feet above the surface of the ground. The match was attached to the end of a copper wire 

 and was about 5 feet above the ground. The case of the instrument was connected to earth by means of a 

 copper wire. The insulation appears to have been satisfactory, and the behaviour of the instrument as tested 

 after the return of the expedition would lead one to expect' no appreciable error from defective insulation. 

 It is, however, not to be expected that the potential acquired by the match and indicated by the electro- 

 meter will give accurately the potential which would exist at the point now occupied by the match, if 

 observer, electrometer, and match were absent. The equipotential surfaces even at the level of the match 

 will be somewhat distorted by the presence of observer and apparatus. In taking the readings the 

 observer always occupied approximately the same relative position, his head well below the level of the 

 match ; if the stand and observer could be simply regarded as earth-connected conductors, the relative 

 values of potential obtained on different occasions would be unaffected, and the error in the absolute value 

 of the potential at the given height would not be large. There is, however, the possibility of an 

 irregularly varying error due to electrical charges on the clothing of the observer. The observer faced 

 the wind while taking the observations, the wind blowing towards him from the instrument, so that his 

 breath or any drifting snow which might have come in contact with him would not reach the immediate 

 neighbourhood of the match. This does not, however, in any way diminish the error due to possible 

 electrification of the observer's clothes. 



For some months observations were also taken of the potential at a height of 15 feet, the match being 

 placed at the end of a rubber-insulated copper wire supported on a pole. These observations were made 

 within a few minutes of the ordinary 5-feet observations. While these observations would be free from 

 the disturbing influence of the observer, one cannot feel the same confidence in the sufficiency of the 

 insulation. 



It was not found convenient to take all the observations at the same place throughout the year. The 

 earlier systematic observations up to May 24, 1902, were taken on shore near "Hut" B (the "absolute" 

 house); those of August 7, 1902, to March 15, 1903, were again taken at the same spot. The winter 

 observations, May 24, to August 6, 1902, and from March 15, to May 8, 1903, were taken on the ice near 

 the ship (about 100 yards away). 



