IOWA ACADEMY OF SCIENCE 
273 
Because of the low capacities to be compared this method requires a high 
degree of insulation for all the parts. But I have been very much sur- 
prised to find how easy it is to obtain almost perfect insulation when 
the neighboring plates are separated by only a few wave lengths of light. 
The only precaution thus far found is that the insulating material should 
be free from moisture and impurities. In general if material once dried 
out is kept a few degrees above the atmospheric temperature, trouble 
due to moisture will be avoided. But for very thin insulating material 
hours of heating may be required, so that it may insulate properly the 
conductors which it separates. 
In these experiments I measured the distances between planes when 
the resistance was of the order of 10 12 ohms, whereas Wood measured 
the distances when the conductivity could be measured by a galvan- 
ometer and a milli-ammeter. Naturally then I should have expected to 
work with planes separated farther than 20 or 30 wave lengths, but I 
was a little surprised to find almost complete insulation at one or two 
wave lengths, when the precautions mentioned were taken. 
In 1908 (Phil. Mag. (6) 18, p. 649) I obtained insulating films which 
were only about 1(P 6 cm. thick. This is nearly a hundred times less than 
certain minimum insulating distances obtained by Wood. One of Wood ’s 
experiments indicated that a coating of oil did not interfere with the 
conductivity. So I tried first a film of turpentine. I evaporated a film 
of turpentine at high vacuum and allowed it to condense on a highly 
polished brass surface. After condensation I was able to bring up in 
vacuum a globule of mercury from below. The globule of mercury 
when earthed and brought up to the film altered the capacity of the 
electrometer system by the amount of the condenser formed. I was able 
to obtain by this method insulating distances of about 6xl0~ 5 cm. As I 
was unable to measure the film thickness by a check method, it was not 
safe to conclude that the electron atmosphere, did not extend into the 
turpentine. 
The first experiments were made with conducting planes of hardened 
steel. They were made and polished by our instrument maker, Mr. M. PI. 
Teeuwen, but the surfaces were not tested at the time optically for 
planeness, and the miscroscope showed them not to be free from scratches. 
However at the conclusion of all my experiments they were tested and 
found to vary from a plane by less than two wave lengths. The area 
was about .1.8 sq.cm. 
The two planes were separated by sulphur particles 'collected from an 
air suspension. Somewhat more than a thousand particles covered the 
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