The measuring should
be done several times over, and the average result taken as the standard.
If the readings all agree, so much the better.
Preparing the Scale.--The next thing is to graduate a scale, which will
most conveniently be established in indelible pencil on a carefully
smoothed strip of white wood 1 inch wide. First make a zero mark squarely
across the strip near the bottom, and at the unit distance above it a
similar mark, over which "One Inch" should be written plainly. The distance
between the marks is next divided by 1/2-inch lines into tenths, and these
tenths by 1/4-inch lines into hundredths, which, if the diameter of the
receiver is four times that of the tube, will be about 3/16 inch apart. For
reading, the scale is held against the tube, with the zero mark level with
the top of the cork plugging the bottom. It will, save time and trouble if
both tube and scale are attached permanently to a board, which will also
serve to protect the tube against damage.
Making the Receiver.--A tin funnel, fitting the inside of the receiver
closely, should be obtained, or, if the exact article is not available, a
longer one should be cut down to fit.
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