448 PROCEEDINGS OF THE AMERICAN ACADEMY 



XXI. 



ATMOSPHERIC ELECTRICITY. 



By Alexander McAdie and Austin L. McRae. 



Communicated May 13, 1885. 



By direction of the Chief Signal Ofiiccr, observations on atmos- 

 pheric electricity were begun at Harvard College, Cambridge, Mass., 

 under the supervision of Professor John Trowbridge, June 1, 1884. 



By permission of the Chief Signal Othcer the following abstract is 

 taken from a report upon the apparatus used, and upon the observa- 

 tions from June 1, 1884, to April 30, 1885. 



Electrometers. 



A Thomson quadrant electrometer No. 26, and a Clifton modifica- 

 tion of the Thomson, were originally used. A full description of the 

 former can be found in the British Association Report for 1867, and 

 also among the reprinted papers of Sir Wm. Tliomson on " Electro- 

 statics and Magnetism," Paper No. XX. The Clifton instrument is 

 a modified form of the Thomson, designed for greater sensitiveness 

 and of less complicated construction. It is not very generally known, 

 and a brief description of it may therefore be given. The essential 

 parts are four large brass or brass gilded quadrants, supported on 

 glass rods of about 10 cm. length. A bifilar suspension carries an 

 aluminium needle, corrugated and shaped like the figure eight. The 

 length of the suspension is about 15 cm. A platinum wire from the 

 needle dips into a glass vessel containing pure sulphuric acid, and 

 coated on the outside and bottom with tinfoil. In the bottom of the 

 case of the instrument, a circular opening is cut, of diameter sufficient 

 to allow the removal of the glass vessel and the metal base on which 

 it rests. In the upper part of the case is placed a small Thomson 

 replenisher. The air within the case is kept dry by small open glass 

 cups containing sulphuric acid. The needle is charged by means of 

 a platinum wire imbedded in a gutta-percha rod, passing through the 

 side of the case, and dipping into the sulphuric acid. 



