INSTRUMENTS AND METHODS. 305 



The self-recording Benndorf electrometer inside the but was also not free from its difS- 

 culties. It has already been explained how the physical laboratory was the coldest comer 

 of the hut. The instruments in it were therefore always colder than the general temperature 

 of the room, especially in the morning when the breakfast was cooked while the instruments 

 were particularly cold after the night. Under such circumstances it was practically impossible 

 to maintain high insulation when the air had free access to the insulators. The insulators 

 on the Benndorf self-recording electrometer are peculiarly badly situated from this point of 

 view, for they are under the instrument and exposed to the air. The insulators holding the 

 quadrants are not important, for the battery which charges the quadrants is quite capable 

 of supplying the electricity for a relatively large leak ; but not so the insulator through which 

 the wire from the collector passes, for unless this insulator is practically perfect the potential 

 to be measured may be greatly reduced. On the Benndorf electrometer there are three insula- 

 tors connected with the system which is charged by the collector : (a) an insulator between 

 the needle and the suspension, (h) an insulator supporting the metal plate on which the 

 acid pnt rests, and (c) the above mentioned insulator through which the connecting wire 

 passes, (a) and (b) are completely within the case while (c) is half within and half without. 

 It was the latter which gave all the trouble and therefore it was done away with 

 entirely. A stout wire was soldered on to the plate carrying the acid pot and a narrow 

 sUt cut in one of the panels of the body of the electrometer through which it freely 

 passed. 



All the essential insulators were now within the body of the electrometer which was 

 supposed to be kept diy by the acid. Still the insulation was far from perfect. Every time 

 the electrometer was opened to adjust or examine the instrument, warm damp air entered 

 the cold interior and the insulation was immediately destroyed. Also the body of the electro- 

 meter was not airtight and even when closed damp air entered. It was at once realised 

 that the only hope was to keep the interior of the electrometer at a higher tempera- 

 ture than the air of the hut. This was done by inserting a small heating coil which 

 was supplied with a current of 'll ampere from the accumulators. After this had been 

 done there was comparatively little difficulty with the insulation of the electrometer and 

 the case could be opened at any time for examination without the insulation materially 

 suffering. 



There is one more cause of difficulty and worry to be mentioned. The polonium collector 

 could not be protected from the formation of hoar frost over its whole surface. This coating 

 of ice reduced its activity greatly and caused an appreciable reduction of the potential regis- 

 tered. This difficulty could not be entirely removed but it was reduced as follows: 



We happened to have with us two identical polonium collectors, these were so arranged 

 that they could be rapidly fLxed to and removed from the top of the collector rod. Whenever 

 meteorological observations were made, every four hours, the polonium rod in use was examined 

 and if it had a coating of frost it was removed and replaced by the other one. The 

 frost-coated rod was then suitably suspended in a canister containing calcium chloride, which 

 dried it and kept it always ready for immediate use. 



When all these changes had been made as the result of bitter experience it was only 

 by constant attention to the electrometer, insulators and collector that satisfactory records 

 could be obtained. 



Two or three times each week the sensativeness of the electrometer and the state of the 

 insulation were tested. To do this the polonium collector was removed, then the electrometer to 

 which the whole insulated system was attached, was connected to a Wulf electroscope and charged. 



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