an Elster and Geitel Electrical Dissipation Apparatus. 479 



unambiguous. And, on the other hand, there are the facts 

 that Elster and Geitel instruments have had an extensive use 

 on the continent, and that there is a larger mass of compara- 

 tive data for a + and a_ than for any other atmospheric 

 electricity element, except perhaps potential gradient. 



It has thus seemed worth while to run an Elster and Geitel 

 apparatus at Kew long enough to obtain fairly representative 

 data, and having done so it has appeared desirable to analyse 

 the results. 



§ 4. Table I. (p. 480) gives results from the three years 

 combined. N denotes the total number of days' observations. 

 a + and a_ are the percentage losses per minute as derived 

 from (1) with n = 0*3, t being almost invariably 20 minutes. 

 a denotes the arithmetic mean (a + +a_)/2, a quantity em- 

 ployed by some observers as a measure of dissipation, g repre- 

 sents the arithmetic mean of individual days' values of a_/a + , 

 while q' stands for (Za_)-h(Za + ), where 2 denotes summation 

 for all days of the month. 



Of the three seasons, Winter is composed of four months 

 November to February, Summer of May to August, and 

 Equinox of the remaining four months. 



The values assigned to the year and the seasons, except for 

 q', are arithmetic means from the monthly values, but in the 

 case of q' they are derived from the seasonal values of a + and 

 «_ carried one significant figure further than in the table. 

 Table I. gives also mean values corresponding to the times of 

 the dissipation experiments for potential gradient P, screen 

 temperature, barometric pressure, amount of cloud (scale 

 to 10), and relative humidity. Meteorological conditions vary 

 so much that it appeared desirable to indicate their character. 

 The pier carrying the dissipation instrument and all 

 uninsulated parts of it were at zero (the Earth's) potential, 

 and the electric field in the immediate neighbourhood was 

 necessarily irregular. The values assigned to P are intended 

 to refer to a spot in the open. They were derived from the 

 electrograph curves by applying factors obtained by reference 

 to absolute observations with a portable electrometer. The 

 mean value of the factor was 1'41. This is higher than the 

 values hitherto employed, recent experiments having shown 

 that previous values were too low. 



The values of a + and a_ are considerably lower than those 

 recorded at most continental stations. The largest values 

 actually measured during the three years were 1*80 for a + 

 and 3*41 for a_. The values obtained for q or q' are excep- 

 tionally large. 



