136 DR. C. CHEEE: ATMOSPHERIC ELECTRICITY POTENTIAL GRADIENT 



shoulder on the long vertical rod, when resting on the top of the ground tube, brings 

 the platform and bamboo rod to the lowest height they can assume. The platform 

 and bamboo can be raised in a few seconds to 1 metre above their original height, 

 by raising the long vertical rod, slipping a pin through a hole in it, and allowing this 

 pin to rest on the top of the ground tube. We thus secure two positions of the fuse, 

 differing in height by exactly 1 metre. To secure that the lower position is exactly 

 1 metre above ground level, use is made of a vertical rod carried by a flat horizontal 

 board. A permanent mark on this rod is 1 metre above the bottom of the board. 

 The board is placed on the ground, and the position of the clamp on the short vertical 

 tube altered until the axis of the fuse, which projects horizontally, comes exactly 

 level with the mark on the rod. The rod and board are then removed and the 

 observation proceeds. The apparatus was designed by Mr. DINES and myself, the 

 material being supplied and fitted by a local builder. The apparatus, though not a 

 finished workshop article, seems to have hitherto served its purpose reasonably well. 

 A recent remeasurement made the 1- and 2-metre intervals each correct to within 

 CT25 mm. 



In actual use larger uncertainties exist in the height, at least in the lower position. 

 The observations are made over a level piece of turf. But a grass surface, even when 

 newly rolled, is not a mathematical plane, and though the grass is kept short some 

 uncertainty necessarily prevails as to the exact level to which zero potential should 

 be assigned. From the mathematical point of view, it would be better to replace the 

 grass by wood pavement or a carefully levelled flag area. But it is at least open to 

 doubt whether on a warm, still day an artificial surface would be as satisfactory as 

 grass. 



4. Absolute observations are made on dry days, usually between 10 and 

 10.30 a.m. The number of monthly observations is usually from ten to twenty. 

 Observations taken at times when the action of the electrograph appears faulty are 

 left out of account. The results, when apparently satisfactory, are entered in three 

 columns. One contains the potentials observed at the 1 -metre level, a second the 

 excess of potential at the 2-metre level over that at the 1 -metre level, and the 

 third the potential as measured on the water-dropper curve. This last is deduced 

 from the length of the curve ordinate and the scale value, the latter being determined 

 from time to time by means of the same portable electrometer that is used for the 

 field observations. 



Calling the three potential data for the same occasion Pj, P 2 , and P 3 , the value is 

 found for each month of the ratios 2Pj/2P and 2P 2 /2P. Eepresenting these by ^ 

 and r 2 , (r^ + r^lZ is accepted as the quantity by which all voltages as measured in the 

 electrograms of that particular month are to be multiplied in order to obtain the 

 corresponding potential gradients in the open. 



It seems customary to assume that the factor for converting curve values into true 

 potential gradient is a constant. It is usually determined once for all from a number 



