The luminous projection apparatus illustrated herewith, when adapted to
a balance of precision, permits of effecting weighings very rapidly. For
the same approximation, the velocity of oscillation becomes five or six
times greater, and, by the method employed, the last centigrammes and
the milligrammes and their fractions are estimated directly, with
immediate verification. As the apparatus is independent of the parts of
the balance, it can be placed on all the existing laboratory balances of
precision.
[Illustration: PROJECTING APPARATUS FOR BALANCES OF PRECISION.]
The modification introduced into the balance consists in the displacing
of the center of gravity of the beam in such a way as to diminish the
sensitiveness, and consequently to obtain a much greater velocity, and
then, by optical means, to considerably increase the amplitude of the
oscillations.
Instead of the oscillations being observed through the microscope, they
are projected upon a divided screen forming a dial, the division of
which is seen by transmitted light.
The apparatus consists of a small achromatic objective placed at the
extremity of the tube of a microscope, in which there is a divided
screen that receives the enlarged image of the reticule fixed upon the
needle. Upon this reticule are projected the rays (condensed by a
powerful lens) that come from a luminous source placed behind the
balance. The focusing is done by means of a rack and pinion.
The luminous source employed is a gas burner with reflector. This is
placed in a walnut box in order to prevent any projection of heat upon
the balance. This burner, thus isolated, is lighted for but one or two
minutes at a maximum, at the end of each weighing. So, on fixing a
thermometer in the cage, we find that no variation, ever so slight,
occurs in the temperature. In order to effect a weighing, the gas being
turned down to a taper, we proceed as with an ordinary balance until the
extremity of the needle no longer emerges from the lower dial. Then we
count the difference of the number of the divisions made by the needle
to the right and left of zero. This difference, multiplied by the
approximate value, in milligrammes, of each division of this dial (value
given by the instrument) immediately gives the number of centigrammes
and milligrammes that must be added to the weights already placed upon
the pan of the balance in order to obtain an equilibrium, to about a
half division of the lower dial.
The value of each division of this dial varies from 3 to 10 milligrammes
according as the balance shows 0.1 or 0.5 milligramme. As the dial has
10 divisions on each side of the central mark, we thus estimate, without
tentatives, the three last centigrammes or the last decigramme,
according to the sensitiveness.
At this moment the doors of the cage are closed, in order to prevent
draughts of air, the gas is turned on by means of a regulating cock, and
the balance is manipulated by first lowering the beam and then bringing
the pans to a standstill. We then read the difference of the divisions
traversed to the left and right upon the luminous dial through the image
of the reticule. The images are reversed upon the dial, but practice
soon causes this petty difficulty to disappear. This number of divisions
indicates the number of milligrammes and fractions of a milligramme by
which it is necessary to shift the counterpoise on its arm in order to
obtain a perfect equilibrium, which latter is verified by a simple
reading. Every half division of the dial corresponds, as to weight, to
the sensitiveness indicated for the instrument.
With a little practice a weighing effected as above described takes but
a quarter or a fifth of the time that it does with an ordinary
balance.--_Revue Industrielle._
* * * * *