The Sun.
London, England, 1858-11-29 00:00:00

AN ELECTRIC C. THERMOMETER.

M. Becquerel, sen., some time ago published an
invention of his, by which the common thermome-
ter was in certain cases superseded by an electrical
apparatus. When it is required to ascertain the
quantity of caloric at different' depths below the
surface of the earth, common thermometers are not
always sufficiently convenient. They must be made
with long tubes of difficult execution, very liable to
break, and requiring great corrections to obtain real
results : two or three metres are moreover the utmost
length which can be given to such thermometers.
If it be required to measure the caloric diffused in
the atmosphere at various heights, there is another
kind of difficulty to overcome ; for when the stratum
of air to be examined is at a greater height than
the house at one's disposal, one cannot read off the
scale, and a thermometrograph will only give
maxima and minima, and not the intermediate
degrees. M. Becquerel's invention consists in
thermo-electric apparatus, by which, under the
circumstances above alluded to, the temperature
may be ascertained to one-tenth of a degree, with-
out any correction at all. Suppose a thermo-
.eletric circuit of iron and copper soldered together
(for there are no fluids used in thermo-electricity) :
if the two soldered points be raised to the same
temperature for a few minutes, a very delicate gal-
vanometer will show the existence of two equal
and opposite currents, the needle remaining at
zero. If the temperature be not the same at both
points of juncture, it must be raised at one of them,
until the needle stops at zero. By this means, 'if
one of the points has a temperature which is un-
known, the observer may calculate it from, that of
the other point, which is known. Now let the
principle be applied to one of the cases above
alluded to, underground observation for example.
Two long wires of different metals, of the diameter
of a milometre at least, are soldered together by
one of their ends, and protected with a coating of
gutta percha. They are then let down into a hole
bored at the place to be explored, and the two un-
soldered ends are brought into the observer's
laboratory, where they are soldered and
put into comniunicafion with the galvano-
meter. Now, if the latter show. that the
temperature requires to be lowered, that is done
by means of ether, which, in evaporating, produces
*old ; if, on the contrary, it is to be raised, this is
done by means of a current of warm air. The tem-
perature requisite to bring the needle to zero will,
by calculation, give the temperature at the point
underground. In much the same way the tempe-
ratures in the upper strata of the atmosphere may
be taken to one-tenth of a degree. A few days ago
M. Becquerel submitted to the Academy of Sciences
a series of experiments made with this apparatus,
in order to ascertain the temperature of plants at
different perions of the day, compared with that
of the atmosphere in a northern exposure, and at an
altitude of 16 metres. The difference of tempera-
ture observable between the latter and the height
of one metre above the soil is very slight. Thus,
the temperature of a maple, about a foot in dia-
meter, and situated in a clump of trees, was, under
the bark, 18.80 degrees from the 30th July to the
30th August ; the temperature taken as above in a
northern exposure was 18.87 degrees ; the maxi-
mum was observable in the maple between 6 and
10 p.m. ; to the north, at 3 p.m., with a difference
in the temperature of the former of 3.0 g deg., in the
latter of 6.14 deg. We cannot follow M. Becquerel
throughout the whole of the delicate experiments
described in his paper ; it is sufficient 'for us to have
explained the principle, and quoted an example.
At the last sitting of the Academy, M. Becquerel
completed the statement of his method by adding
that the temperature at an altitude of 16 metres
was taken with his electric thermometer, protected
from direct radiation of the sun by a polished silver
reflector.