242 IOWA ACADEMY OF SCIENCE Vol. XXIV, 1917 
about one milliampere difference between the two currents with 
the slightly greater current flowing from zincite to copper. 
Curves No. 4 show the behavior of the same contact when rever- 
sals of the electromotive force are made at the end of every 
minute. The general effect of the quick reversals is to lessen the 
difference between the two currents. 
Curves No. 5, figure 42, show a zincite-copper contact in vacuo. 
In this particular case there is a difference of about two mil- 
liamperes between the two currents with the greater current flow- 
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Figure 41 
ing from zincite to copper. Usually the difference between the 
two currents was less than a. milliampere in vacuo. 
Immediately after letting in the hydrogen-sulphide there was 
an increase in the apparent resistance of the contact, causing 
the current to fall from seventy-four milliamperes, zincite to 
copper, to sixty-nine milliamperes, zincite to copper. This effect 
is characteristic of hydrogen-sulphide and never failed to appear 
whenever the gas was let into the vacuum tube. Curves No. 6 
show the action of the contact shortly after letting in the hydro- 
gen-sulphide. The difference between the two currents is now 
about five milliamperes with the greater current from zincite- 
