208 NINE THERMOSCOPES 4/-4 *8th C. r 198 ) Fig. 184
Length about 240, diameter of bulbs 30—35.
Nine thermoscopes containing various liquids, all contained in a mahogany box, with
compartments made to hold twelve U-shaped and four Z-shaped tubes. The latter
have the short arm bearing the bulb bent at right angles to the main tube, but on
opposite sides. The glass tubes have paper labels with the contents written in ink. As
far as they can be read, they are:
U-shape: ol. Tcreb; sp. nitri; acidum rigel; ol. Bergum; sp. salis amm.; naphtha
(2 tubes).
Z-shape: ol. aurant; mercury (unlabelled, but obviously this metal).
Thermoscopes serve to demonstrate the nature of evaporation and boiling; sec
Cat. 207.
209 DIFFERENTIAL AIR THERMOMETER: Leslie-Rumford
'/4 i9thC. (175) Fig. 185
Overall height 365, length of scale 240, diameter of bulbs 49.
A glass capillary tube is bent to the form of a wide U-shape, with bulbs at each
end. This is mounted on a 1-shaped stand of brass, with a silvered horizontal scale
marked 60—0—60 by ones. Leslie’s degrees were milligrade, or a tenth of a degree
Centigrade, but Runtford’s were arbitrary. At the middle of the scale the tube contains
a marker bead of liquid. If one bulb is placed in the path of radiant heat, the
air contained in it will expand, and move the marker. Both bulbs will be equally
affected by local heat, hence the differential property of the device. Although not
accurate, it is a useful demonstration instrument.
Devised by John Leslie (1766—1832) for his experiments on radiant heat (see his
photometer Cat. 210, hygrometer Cat. 211, and cubes Cat. 212), the differential air
thermometer was also claimed by Benjamin Thompson, Count Rumford (1753-1814).
The principle, however, derives from Galileo’s air thermometer; see Cat. 207. Rum-
ford read a paper on heal to the Royal Society on 2 February, 1804, when he
referred to: “ ...an instrument I contrived for measuring, or rather for discovering,
those very small changes of temperature in bodies. The instrument, which I shall take
the liberty to call a Ihermoscope, is very simple in its construction. Like the hygrometer
of Mr. Leslie (ns he has chosen to call his instrument) it is composed of two
glass balls,...” . The differential thermometer with the horizontal scale, as here, is
the so-called Rumford type, while the vertical scale version is the Leslie type.
In about 1820, both R. B. Bate and R. & G. Knight offered for sale Leslie’s
differential thermometer at three guineas.
Leslie-(1800); (1804) 9-11; Rumford (1804) io if IV, fig. 2.
210 PHOTOMETER: Leslie’ s 1802 (1128/1) Fig. 186
Probably by Miller & Adie, Edinburgh
Overall height 171, diameter of base 84, diameter of bulbs 12.5.
A capillary tube is bent into a U-shape, and at the end of one arm is a bulb of clear
glass, while (he other terminates in a bulb of black glass. A small quantity of dark
coloured liquid is held within the tube, which is supported with an ivory scale reading
from 0-150 by ones, on a turned, walnut base. When put in a beam of radiant heat,
the black bulb will absorb heat and expand the air within it, thus moving the liquid
across the scale, and so giving the measure to the intensity of radiation. Local changes
will affect both bulbs equally, and so the differential thermometer may be a sensitive
detector for light and heat.
The photometer was designed to measure the intensity of sunlight, and for experiments
on reflection and transmission of light. John Leslie (1766-1832), knighted in
the year of his death, was a mathematician and natural philosopher. He became