A NEW OR IMPROVED METHOD AND APPARATUS FOR MEASURING AND REGISTERING RADIANT LUMINOUS AND CALORIFIC ENERGY
Year: 1883

page_text: PROVISIONAL SPECIFICATION left by William Siemens at the Office of the
Commissioners of Patents on the 17th September 1883.
WILLIAM SIEMENS of N° 12 Queen Annes Gate Westminster in the County
of Middlesex, Knight, "A NEW OR IMPROVED METHOD AND APPARATUS FOR
MEASURING AND REGISTERING RADIANT LUMINOUS AND CALORIFIC ENERGY."
In comparing lights by means of the photometers in common use, a difficulty is
experienced in determining the relative luminosity when the lights are of different
intensity and quality. Thus it is difficult to compare the effect of an electric arc
with that of a standard candle, owing to the preponderance of blue light in the
former.
My invention relates to a method and apparatus by which I am able to determine
separately the luminous and other forms of radiant energy emanating from a
radiating source, according to their relative magnitudes, and to express them in
Watts or other unit rates of energy.
For this purpose I mount a prism, which may be simple or compound, or its
equivalent, adjustably in a frame. When prisms are employed, they should be of
material that allows the rays of all degrees of refrangibility to pass with the least
obstruction, and for this purpose rock salt prisms coininend themselves. Instead
of prisms, finely ruled metallic surfaces, sucli as are known as Rutherford gratings,
may be employed. At a certain distance behind the prism or its equivalent, I
arrange between two parallel surfaces a series of bays or cavities, preferably of
parabolic form, with good reflecting surfaces, such as speculum metal, silver br
mirror gles. These cavities are arranged in a curve so that there is equal
distribution upon them of the rays that pass through the prism from au exterior
source of light. In each cavity is placed the black bulb of a thermometer, the stem
of which projects outside the casing, so that its indication can be observed against
a scale at its back. The blackened bulos of these thermometers are by preference
made long and narrow, so as to absorb us much as possible of the radiant energy
falling into each cavity, and to become proportionally heated.
When a source of light is placed at a given distance in front of the prism, the
rays of energy that pass through a slit or aperture in its front, become distributed
D

page_text: according to their relative refrangibility, the invisible calorific rays being directed into
the cavities near one end of the series, and the luminous rays red, yellow, blue and
violet in their proper order, into the other cavities. If the source of radiation is
of comparatively low intensity, heat rays will predominate; but if it be of high
intensity, as for example an electric arc, the proportion of luminous rays will be
much greater and comprise yellow, blue, violet and ultra violet in proportions that
will be indicated by the successive thermometers. The thermometers are made to
have small range, the same thermal capacity and similar scales, and in order to
obtain comparative results, the exposure of the apparatus to the source of light
should, in all cases, be the same. At the end of a fixed period of exposure, say
five or ten minutes, the simultaneous indication of each thermometer in the series
is noted down, and, losses of heat by conduction or otherwise being predetermined
and allowed for, the increase of temperature of each thermometer will signify the
relative amount of radiant energy passed into its cavity as heat and luminous rays,
according to their respective refrangibilities.
A permanent record of the relative temperatures taken simultaneously may be
obtained by means of a photographic arrangement consisting essentially of a roll of
sensitive paper placed inside a dark chamber, with a slit parallel to its axis opposite
to the line of therm meters, of which when light is thrown upon them, an image
will be produced on the sensitive paper, showing the indication of each thermometer.
A lens may be employed in front of the slit to concentrate the light upon the
paper, and thus reduce the time of exposure. After each exposure, the slit is closed,
and the roll of paper advanced to make room for another impression. The image
on the sensitive paper may be fixed in the usual manner. The source of light may
be provided with reflecting arrangements, so as to direct a large proportion of the
radiated energy upon the apparatus.
Instead of thermometers, thermo-piles may be employed to indicate the heat
produced in each cavity, or the heat may be ascertained by means of a filament of
blackened iron or other conductor, the electric resistance of which, increasing with
the temperature, can be accurately measured by the electric balance.
The use of the apparatus above described as a Pyrometer is based upon the
known fact that, with the intensity of a source of radiant heat, the proportion of
luminous and non luminous rays increases, and this proportion, as determined by
the apparatus, furnishes a reliable indication of the temperature: