Mr. F. S. Spiers on Contact Electricity. 



73 



readings, however, not much stress must be laid on these 

 results. 



§ 5. In designing a new apparatus, Fig. 2. 



the following points were specially 

 kept in view: — (1) The desirability 

 of being able to measure the actual 

 value of the Volta effect. (2) The 

 absolute necessity of being able to 

 heat strongly the contact couple in 

 the highest attainable vacuum, (o) 

 The necessity of having high insu- 

 lation, in consequence of the small 

 capacities dealt with. It will be found 

 subsequently that the exigencies of 

 (2) reduced the capacity still further 

 than would otherwise have been the 

 case. 



To fulfil condition (2) it was neces- 

 sary to make the apparatus of hard 

 combustion-tubing ; but as such glass 

 is extremely difficult to work, and 

 as it would have been exceedingly 

 troublesome, if not impossible, to seal 

 glass tubes or platinum wires into it, 

 it was decided to make the apparatus 

 in two parts. This had the further 

 advantage that the plates could be 

 taken out and examined whenever 

 desirable, which would not have been 

 the case had the tube been entirely 

 sealed up. 



The apparatus is shown in fig. 2. 

 A, A the lower half of the tube is 

 made of the hardest combustion- 

 tubing 1 J in. in diameter. It fits into 

 the upper portion B, B, made of softer 

 tubing, at the very carefully-ground sectional elevation 



glass joint C. The joint is sealed in 

 mercury contained in a glass vessel, 

 which slips over the rubber ring I). 

 The two dissimilar metals E and F 

 form the contact couple. Their di- 

 mensions are f in. by 5f in. They are 

 attached to the aluminium spindle Gr 



by means of the zinc lugs H, H, to which they are screwed. 

 The spindle passes through slits in these lugs, and they are 



[Ji-jfli ' PLAN OF 

 U-i"-V| ! PLATES 

 4— Ji^-*i 



