76 
Transactions of the Boyal Society of South Africa. 
observations were not taken with the object of elucidating this point, but 
the fact remains as stated, and this was the case whether the rainfall was 
heavy or light. The author has shown that in hot, dry weather the 
surface of the ground in the Orange Free State is apparently charged with 
positive electricity. The real origin of the fine-weather potential gradient 
must be considered as not definitely known at present. 
Method of determining the Potential Gradient. — If an insulated 
conductor is placed in an electrostatic field, it has a tendency to take 
the potential of that portion of the field in which it is placed, but of 
course no permanent charge can be obtained by it unless either 
positive or negative electricity can escape from it. To facilitate this 
escape several methods are employed, such as attaching a burning 
candle or a piece of smouldering tinder to the conductor, or by means of 
a stream of water falling from a fine jet, or, most convenient of all, by 
coating the conductor with a radio-active preparation. In the latter case 
the conductor may be made small, and also may be moved quickly from 
one point to anotlier in the field. Id order to measure the potential 
gradient, the " collector " must be placed at a definite distance above 
the ground, because the actual potential observed depends upon the 
heicjht at which the collector is fixed, but, within certain limits, 
is a constant quantity over a short interval of time, and the 
fraction, when the potential is measured in volts and the height in 
metres, is usually called the potential gradient. In order to measure 
the potential, the collector is connected to some form of electrometer of 
which the gold-leaf electroscope, with a reading microscope, is undoubtedly 
the most convenient if the instrument is not to be self-recording ; but if 
a permanent record is required, then some form of quadrant electrometer 
is usually employed. 
The instrument in the present work is a form of the Kelvin instrument 
as modified by Benndorf, and supplied by Castagna of Vienna. It consists 
essentially of a quadrant electrometer, to the needle of which a long 
pointer is attached. The pointer moves over a band of paper, and at 
every minute or half-minute it is pressed into contact with the paper, 
a piece of thin " carbon " tissue coming in between so that a mark 
is impressed upon the paper band. The band is carried over by clock- 
work, which runs for eight days without rewinding. A three months' 
record can be taken on one roll of paper. The record thus appears as 
a series of dots at minute or half-minute intervals, and by connecting 
these dots a daily curve can be readily traced. There is a length of 
96*4 cm. for each day's record, so that it may easily be measured up. 
In using the instrument, one pair of quadrants is put to earth, the 
other pair joined to the collector, and the needle kept at a fixed potential 
* Phil. Mag., May, 1912. 
