between Mercury Surfaces. 483 



accumulator and measuring-galvanometer, allowed the electro- 

 motive force to be ascertained and compared with that of a 

 standard cell. 



The readings were found to depend greatly upon the energy 

 with which the glass scrapers were parted, and upon the pre- 

 cision with which the discharge-key was released at the mo- 

 ment of scraping. The electromotive force between two sur- 

 faces of pure mercury which had stood a short time under dis- 

 tilled water was found to be as follows : — 



A scraped, B scraped, 

 B quiet. A quiet. 



0-033 volt. -0-046 volt. 



0-045 „ -0-020 „ 



0-041 „ -0-032 „ 



When very dilute sulphuric acid was substituted for the 

 distilled water, the average readings became higher, as fol- 

 lows : — 



A scraped, B scraped, 



B quiet. A quiet. 



0-067 volt, -0-064 volt. 



0-065 „ -0-086 „ 



0-032 „ -0-041 „ 



0-068 „ -0-077 „ 



The last experiment was varied by scraping one of the sur- 

 faces backwards and forwards with a glass scraper, and obser- 

 ving the current by a delicate mirror-galvanometer with a 

 very high resistance in the circuit during the scraping, and 

 at intervals of fifteen seconds after ceasing to scrape, until 

 the two surfaces had resumed an equality electrically. Whilst 

 the one surface was scraped, the other surface was at rest, 

 The deflection showed in all cases that the scraped or disturbed 

 surface corresponded with the zinc of a galvanic couple. It 

 was also found that, however clean and bright the surfaces 

 might appear, the scraping of one of them was always able to 

 establish a current of electricity. 



Surface A scraped gave a steady deflection of 130 divisions 

 to the right, representing an electromotive force of 0*036 volt. 

 When the scraping was discontinued, the deflection fell gradu- 

 ally as follows : — 



After 15 seconds 90 divisions right. 



„ 30 „ 80 



» 45 „ 50 „ 



„ 60 „ 30 



» "'0 „ 20 



„ yo „ io 



2 12 



