interference pattern is done by the observer by
remote control; the micrometer (2) of the zoneplate
is read on a clock (4). The total arrangement is
fixed within a frame of four pillars standing on a
sledge allowing a small horizontal movement (also
by remote control) for easier pointing. The total
instrument is placed on a triback on a heavy mobile
industrial tripod. The height of the instrument is
approximately 50 cm.
The windows in the pipe have a diameter of 42 mm
thus allowing for a fiat liquid surface in the centre.
The arrangement of the needle did not appear satis
factory because of adhesion and corrosion problems.
After much experimenting the following design
gave the best results. The needle was replaced by a
cap with a flat bottom and a float (of a carburettor)
was placed in the window (Fig. 4). It consisted of a
barrel (diam. 21 mm) with a vertical bar in the
centre of which a platinum wire of 0.2 mm length.
Underneath three horizontal bars placed under an
angle of 120° made the float automatically main
taining a central position (Fig. 5). Such a float was
placed in each window. It was then necessary for the
alignment to standardise them for the reduction of
differences in height of the platinum wire above the
liquid level. It was found that the precision of mea
suring the downward movement of the micrometer
until the cap touched the platinum wire could not
only be kept steady, but was also the highest of all
possibilities tried. The s.d. derived from the counter
readings was 2.1 pm.
3 The observations; precision
During the measurements, airpressure and air-
temperature (wet and dry) and the internal tem
perature of the liquid at every window (by built-in
thermocouples) were registered. The evaporation
was checked by separate regular readings at one of
the floats (the other windows remaining covered)
during a number of extended periods when air-
pressure and temperatures were stable. The amount
of evaporation was approximately 0.03 pm per
minute.
The order of observation decided upon was the
following, [t was not possible in the present
arrangement to fix the two terminal points indepen
dently and measure the height above the liquid
level (the ideal situation as it should be effected in
the industry). The position of the zoneplate in the
"laser" line was determined instead above the
terminal floats nos. 1 and 6 by the mean of 4 series
of 10 settings. These two points were then assumed
to be fixed with a s.d. of 1.1 pm.
It was decided that the influence of the evapora
tion for the 10 minutes necessary to take these
measurements and the shifting of the tripod in be-
Fig. 3.
Fig. 5.
Fig. 4.
140
ngt 76
