The Mankato free press. [volume] (Mankato, Blue Earth County, Minn.) 1879-1924, 1903-12-18

An Interesting Article From ths Fen
of B. r. SteGman.
From WodpaSdav,c Jloilv

To the Editor of The Free Press:
Last Sunday morning we had all kinds
OF temperature, running from about 20
to % degrees below Zero Smith is dead
sure his thermometer is good one, and
insists that it was % degrees below;
Jones knows his thermometer is as good
as any and it was 20 degrees below. 1
have been asked to explain the why of
this difference, and will try. I have
handled hundreds of thermometers, have
seen them in process of manufacture,
and have experimental in many ways
with them. 1 think I know something
of them.
Many of the causes of variation in
thermometers will be understood when
we explain the process of manufacture land
when we know that the manufacturers
price for ordinary thermometers runs
from fifty to seventy five cents dozen
we may not expect much accuracy. I
have seen dozen of them in a bunch,
no two reading alike, with 10 degrees
variation between the highest and low
est. But to their making. A workman
has "blowing rod" say four feet long
by an inch in diameter; he puts one end
in molten glass, revolves it until he has
ball of sufficient size adhering, with
draws it quickly, blows in the other end
forming an air bubble in the glass. An
other workman is ready with a rod with
adhering hot glass but with no bubble.
The two balls when brought in contact
instantly unite; the workman now walk
rapidly away from each other, and the
glass spins out in a rod, the air from the
bubble forming bore the whole length.
This glass rod is used for thermometers.
An absolutely perfect thermometer
should have an exactly uniform bore it's
entire length. But the bore of these
glass rods is not uniform. The bore is
measured by injecting small quantity
of mercury, and measuring the length of
space it occupies. It is then moved
along and again measured. If the bore
expands the mercury occupies less space;
if it contracts, longer space is occu-
pied. From the tubes of nearly uniform
bore the better grade OF instruments are
made; the remainder is used for low
grade. For the purpose of this article
it is not necessary to &C into the making
and attaching the bulb or the filling with
mercury or spirits. But another point
must be mentioned. Those tubes and
bulbs contract for about two years after
being made, and if thermometer should
be made marked today, two or three
years from now it's readings would be
from two to five degrees higher than to-
day. A cheap thermometer, having a
variable bore has these peculiarities. It
is never correct at all readings. 1t may
be correct at one or more temperatures
or readings, too high at others, and too
low at yet others. Great care is used in
marking clinical or medical thermome-
tiers. Their scales run from plus 95 de-
grees to plus ITO degrees-occupying
about one and one-half inches of the
tube. Not one in hundred is correct
at all points. Each one has certificate
giving it's errors. I have handled thou
sands of them, and am confident never
saw dozen with no errors.
Some of my experiments here in Man.
kato may be of interest. The winter of
1876 was very cold and stormy; eight
times it went to 40 degrees or lower.
Prof Bechdoldt and myself formed a
private weather bureau. We had sta
lions on the Agency hill, at the Normal,
at my residence on South Front street,
and one near the Milwaukee depot. By
numerous trials we demonstrated that
when it was excessively cold, but with
no wind, it registered lowest at the de-
pot; on south Front about two degrees
higher than at the Normal, an additional
rise of two degrees, and on the hill about
three degrees above the Normal; making
usually about seven degrees lower at the
depot than on the hill. But when we
had blizzardy wind there was but very
little variation. We found that if we