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independent of the difference in pressure to be adjusted. Therefore 
pressures differing but little from the atmospheric can be 
employed, which makes circumstances more natural for the plant. 
Fig. 1 refers to a moment, when the difference in pressure is being 
adjusted, so the water is falling in M, and rising in M, and the 
pulley is moving as the arrow indicates. Now there turns on the 
same shaft a second pulley and over this a cord passes, carrying 
at either of its extremities a n-shaped fork of nickel wire. Such 
a fork moves, when the pulley terns, in a vertical direction, with 
each of its limbs in a glass tube, at the bottom of which is mercury, 
which has been inserted into a circuit. 
If the fork 8 dips into the mereury of contact C,, circuit I is 
closed and the current passes through the solenoid 8, of the auto- 
matie electric mercury-switech W (upper half of Fig. 1). The core 
of soft iron is attracted, the switch is changed over and a.o. one of 
the insulated iron forks dips into the mercury cups of contact C,, 
in consequence of which circuit III is closed, the electro-magnet 
opens the cock K,, the porometer is brought in connection with the 
pump and the water rises in M, and falls in M,. The air-rarefaction 
and with it the movement of the water and of the pulley continues 
till the fork 9, which is sinking now, reaches the mercury in C,, 
for in consequence of that circuit II is closed, solinoid 8, works 
and, the switch changing over, circuit III is broken, the porometer- 
space is closed by the clamp and the current of air passing through 
the stomata rebegins the adjustment of the difference in pressure. 
It is evident, that the degrees of air-rarefaction between which 
we want to work, can be regulated by altering the place of the 
mercury-contacts along their stand. 
The preceding may be summarized as follows: 
There are two alternate periods in the working of the apparatus: 
Period I, of indefinite shorter or longer duration, in which the 
position of the switch is such, that the current through the electro- 
magnet is broken; the porometer-space consequently is closed by the 
cock and the pressure is increased by the current of air through the 
leaf from the lowest fixed limit to the highest. 
Period LT, of a short duration, in which the position of the switeh 
is such, that the current passes through the electro-magnet; the poro- 
meter-space consequently is connected with the pump, which dimi- 
nishes the pressure again to the lowest limit. 
Each period begins with a contact, made by one of the forks (taking 
part in the movement of the water in the manometer) through which 
the switch is drawn in the required. position. 
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