Earth- Aii- Electric Currents. 719 



where the earth's normal electrical field was undisturbed. 

 This, however, was quite impossible in Simla, for the town 

 is situated on a ridge of the Himalayas, and there is no 

 single plot of ground within many miles of the town on 

 which the field is not disturbed by the configuration of the 

 surrounding hills and valleys. The Simla Meteorological 

 Office is at present located in a house originally built for a 

 dwelling on a spur branching off the main ridge on which 

 the town is built. The house is situated on the extremity of 

 the spur, and the ground slopes down for many hundreds 

 of feet at a very steep incline on three sides of it. There is, 

 however, between the house and the main ridge a small 

 piece of level ground (about 100 ft. x 50ft.) on which a tenuis 

 court has been made. The tennis court is surrounded by 

 stop-nets about ten feet high and several trees 30 or 40 feet 

 high are growing against them. The branches of the tires 

 which were growing over the court have been cut away so 

 that the court itself is fully exposed to the sky. The middle 

 of this court was the best site which could be obtained, and 

 a small tea-house built close to the stop-nets on one side 

 afforded a suitable shelter for the instruments. Thus the 

 plate was exposed in the middle of a small open space, the 

 electrical field of which was considerably affected by tin; 

 near trees and stop-nets :i- well as by the irregularities of 

 the surrounding hills. 



The exposed plate itself consisted of a wooden frame 

 (285 X 570 CUIS. = 17 metre-) on which canvas \va> tightly 

 stretched. The canvas was covered by a sheet of brown paper 

 made conducting by means of a thick coating of blaeklead. 

 In this way a large conducting surface was obtained which 

 was at the same time light and strong. The sheet was insu- 

 lated on six sulphur insulators which raised the upper surface 

 of the sheer about looms, above the general level of the 

 ground. It would have been much better if the sheet could 

 nave been actually at the ground level, but as this would 

 have necessitated cutting up the surface of the tennis court 

 it was not possible to do it. It will be Been from the above 

 that the exposure of the plate was far from that of a piece * f 

 the earth's surface exposed to the normal potential gradient; 

 but it was the best which could be obtained under the 

 circumstances. 



R suits. 



Let each cm. 2 of the exposed plate be receiving a charge 

 from the air at the mean rate of x els. units per second. 

 Then in a given time t the whole plate will receive from 



