MEASUREMENT OF ATMOSPHERIC ELECTRICITY 
ment is particularly important. Such treatment includes 
high polish, drying, dust removal, paraffin coating m 
some cases, and protection against direct sunlight. 
When working in the open air, careful maintenance of 
the dryness of the dielectric surfaces by drying with 
hot air or by electrical heating is essential. The relative 
humidity on the msulator must not exceed approxi- 
Fie. 2.—Wulf’s bifilar electrometer. 
mately 80 per cent. Figure 4 gives one example of a 
suitable form of open-air insulator which has held up 
well in practice. 
To combat insects and spiders it is helpful to paimt 
the metal parts about the air gap with an adhesive 
substance (fly-paper glue) and also with aromatic insect 
repellants. To eliminate air-borne spider threads in 
summer and fall, mechanical methods must be applied. 
In all electrostatic measurements it is essential to 
145 
maintain good grounding of all parts to be maintained 
at zero potential. In general, grounding to a water pipe 
is satisfactory. It is particularly important that all 
parts to be grounded are connected to the same ground 
Fig. 3.—Wulf’s unifilar electrometer. 
CaClo 
for mechanical load 
large 
(antennas and the like). 
Fre. 4.—Open-air insulator 
connection. All other leads must be protected from 
induction by shielded cables. 
With high electrometer sensitivities undesirable dis- 
turbances may appear as a result of certain unforesee- 
able phenomena in the insulating dielectric material, 
such as the formation of deposits and polarization 
phenomena. Direct creep of charges across an insulator 
