202 Dike — Recent Observations in Atmospheric Electricity. 



was carried designed to give a rough calibration on board ship, 

 but its results, after a test at Sitka, and another, an unsuccessful 

 one, at Honolulu, were judged too rough to be relied upon. 

 It was possible to obtain a good calibration at Christchurch, 

 New Zealand, in the electrical laboratory of Canterbury College 

 in January of 1908, but there was no further opportunity 

 until the return of the instruments to this city in July, when 

 a calibration was carried out at the Bureau of Standards, agree- 

 ing remarkably closely with the curve obtained at Christchurch, 

 showing that at least during the last half of the voyage the 

 values had remained practically constant, that is within three 

 per cent, which is well within the accuracy of reading at sea. 

 A comparison of these values with those obtained at Sitka 

 indicates a gradual increase in sensitiveness, which goes on 

 at a diminished rate between Christchurch and Washington. 

 Through unexpected good fortune I was able to use the same 

 pair of leaves throughout the voyage uninjured, though the 

 electroscope had to be opened to clean the insulation, thus 

 exposing the leaves to air currents and other dangers of damage, 

 nearly every time the instrument was used. 



An electroscope is far from being a satisfactory instrument 

 for use at sea, but it is the only form of electrometer now avail- 

 able that would work at all, unless some form of the "string 

 electrometer" may be found satisfactory. 



C. T. R. Wilson of the Cavendish Laboratory has made some 

 interesting observations of the earth-air current with a very 

 different instrument, designed by himself, by means of which 

 he is able to measure the current from a test plate which is 

 kept at zero potential, that is, at the potential of the surface. of 

 the earth, thus approximating closely to the actual conditions. 



The case of a gold-leaf electrometer is kept at a constant 

 potential by means of a quartz Leyden jar, while the gold leaf 

 is attached to the rod bearing the test plate, a blackened brass 

 disk 7 cm in diameter surrounded by an earth-connected 

 guard ring. A cylindrical cover rests upon the guard ring 

 and shields the test plate except when it is to be exposed 

 to the influence of the earth's electrical held. During exposure 

 the test plate and gold leaf are maintained at zero potential by 

 means of a compensator consisting of a cylindrical condenser 

 of which the inner conductor is a metal rod connected to the 

 gold-leaf system, the outer condenser being a brass tube main- 

 tained during any series of observations at a constant negative 

 potential by means of a quartz Leyden jar and capable of sliding 

 parallel to its length to give a variable capacity. A scale is 

 attached to give the position of the outer cylinder relative to 

 the inner and this scale is calibrated to determine the change 

 of capacity corresponding to one scale division. Thus the 





