Antecedentes católicos de la democracia americana

A photoelectric spectrophotometer was constructed which was of simple design, yet which was capable of detecting low levels of light intensity over the entire visible range, and of employing small slit widths.

Essentially, the instrument consisted of two parts, (a) an optical system, and (b) a photoelectric

multiplier cell with "accompanying potentiometer and power pack (see fig,,'X).

A source of white light is focussed on the entrance slit of the monochromoitor which was adapted from a Hilger constant deviation spectrometer by substituting a variable

slit for the eyepiece. By turning the wavelength drum, the diffracting prism is slowly rotated in such a way that the spectrum formed moves across the exit slit into v/hich it is focussed by a lens. The second slit,

therefore, transmits a narrow^ band of approximately

monochromatic light, the centre wavelength of which is given by the reading on the wavelength drum. The cells

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a smooth sliding carriage, so that either cell may he placed, in the path of the monochromatic ray falling on to the sensitive surface of the multiplier cell. After amplification, the photoelectric current is

detected hy a sensitive mirror galvanometer and

measured hy readings of the spot on a centimetre scale. The ratio of successive readings for solution and

solvent gives a measure of the transmission (]/I ) of the solution.

It is advantageous to have the optical cells situated at the exit slit of the monochromcCbor, since the low intensity of light passed hy the monochromator is insufficient to cause appreciable photo-chemical decomposition of the solutes. In contrast to this, the photo-chemical decomposition of ferric thiocyanate was marked when solutions of ferric thiocyanate were placed before the entrance slit of the monochromator, despite the use of suitable filters, and of heat

absorbing glase.

Light Source. ««Itl *( «« M'U HMIIU wéMCUI MM ■ 1.1* I# The source of white light was a 500 watt Osram projection lamp, run from a 230 volt constant

voltage transformer. A plate of Orookes heat absorbing glass (N 1 1 9 ) was positioned between the lamp and the

solutions and optical system while suitable filters (Ilford H 5 5 8 Bet and others) were placed behind the heat absorbing glass to diminish the effect of stray light from the prism.

Monochromator. Both entrance and exit slits of the > monochromator were calibrated to be set at any desired

width, and optimum settlings were determined for various band widths isolated. Incorporated in the entrance slit was a variable wedge for regulating the intensities of the beam of light entering or leaving the monochromotor. This could otherwise be done with the aid of neutral density filters placed before the entrance slit.

Optical Cells and Cell Carriage. The optical cells were fused U-bends (Tintometer - GIF 2If., SCL 4), and were one centimetre in width. These were held in a

sliding carriage to which they were firmly attached by means of a springy brass clip. The carriage could easily be moved to one or other of two fixed positions bringing each of the cells, in turn, directly into the path of the monochromatic beam. Mechanical perfection of the cell mounting was ensured, and thus, also,

reproducibility and equivalence of the cell positions. »

Photoelectrio Oeil. The photoelectric cell was a 9-"Stage ”Gintel” multiplier cell (¥A 20, Serial No* A 3 6 3 ) in which the primary current is amplified hy

secondary emission. The overall voltage was supplied hy a maints rectifying unit giving an output voltage of 1000 volts. Fig. I illustrates 'both this power unit and the potentiometer providing the potentials for the individual stages in the cell. The characteristics of the photocell are given as

3 0 - 8 0 p,a/L' Cathode Sensitivity

1 0 0 0 - 5 0 0 0 total multiplication when 1000 volts overall.

The spectral sensitivity curve provided by the makers is sketched, fig. II. The photocell was contained in a dark metal box and mounted on a bakelite base which afforded

complete insulation to the electrical leads passing through The potentiometer was mounted in a bakelite container

where it had access to circulating air.

palvanometer. A sensitive mirror galvanometer (Cambridge D* arsonal approximately ohms resistance) detected the

small photocurrent, used in con*junction with a scale graduated in centimetres.

The monochromator, cell carriage and photocell were all joined in one lightproof unit firmly fixed to an optical bench. The positions of the other components of the optical system could be adjusted to give finer focussing on the entrance slit,

A similar spectrophotometer was constructed at an earlier date in which the spectral region was isolated by means of a Bellingham & Stanely monochromlator. This

instrument v/as found to be useful in work of a preliminary nature but was less satisfactory than the instrument just described in several respects, being less refined and, in particular, since it had an arbitrary wavelength scale. In consequence it was set aside in preference to the present instrument which was used in obtaining all the data to be given.

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