Vacuum Gold-leaf Electroscopes in Rarefied Gases. 751 



brouglit near, being protected by the presence o£ the wire- 

 gauze cover, as was shown by Faraday, in his Experimental 

 Researches. 



A great variety o£ experiments were also tried, but need 

 not be described in detail here, on the effect of screens of 

 metal, glass, mica, cardboard, paper, &c, also of liquids con- 

 tained in a parallel-sided cell, and, lastly, of vacuum ; the 

 vacuum between the walls of a Dewar vessel for holding liquid 

 air being used for this purpose. 



To ascertain how far the phenomena observed might be 

 attributed to heating and cooling effect, the region of air 

 surrounding the radium clock was carefully explored with 

 the help of a very sensitive thermo-electric junction. The 

 result of this experimenting was to show that the thermal 

 radiations, which were almost too slight to be detected 

 with the thermo-junction, could produce very marked effects 

 in causing disturbance of the gold leaves. 



It was now determined to construct a special gold-leaf 

 electroscope for the purpose of investigating the phenomena 

 which these preliminary experiments pointed to ; and, after 

 several small electroscopes had been made, a convenient 

 instrument was obtained, which may be described as follows,, 

 and of which fig. 3 (p. 752) is a drawing. The essentials 

 aimed at are : (1) convenient dimensions ; (2) perfect dryness 

 of the interior ; (3) high exhaustion. 



The electroscope is contained in an enclosure, made from a 

 piece of glass tubing, 1^ inches in diameter, drawn out at 

 both ends to form tubes e, e\ fig. 3. A piece of thermo- 

 meter tubing t is sealed on to the neck e' , and the aluminium 

 wire m, n, supporting the gold leaves, passes down the centre, 

 and is held in position by the tube t, the joint between the 

 glass and aluminium wire being made vacuum-tight by filling 

 the cup c with " siegelwachs/' 



Two pieces of lead-foil a, a, about 1^ inches long and 

 1£ inches wide, are cemented on to the outside of the glass 

 enclosure opposite the faces of the gold leaves, and are joined 

 together by strips of lead-foil x, y, which completely encircle 

 the glass enclosure. 



A copper wire iv is fastened over the upper strip of lead-foil 

 to allow a connexion to the metallic coating to be easily made. 

 The tube e is connected to a series of drying-tubes, and to a 

 pair of five-fall Sprengel pumps, to maintain a high degree of 

 exhaustion in the enclosure. 



With the instrument thus described, the following expe- 

 riments were carried out, both while the electroscope was 

 still connected to the pumps and after it was sealed^oft'. 



