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Volume 71, 1942
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2. Calibration of Photoelectric Amplifier Bridge.

In 1939 a standard mercury quartz are lamp (Hanovia Wehnelt electrode type 16100) was made available by Dr. E. R. Cooper, and with it calibration measurements were made on the p.e. amplifier bridge and on the balanced thermopiles. These measurements, which are reported herewith, illustrate the methods employed and the checks obtained, but, unfortunately, the operating conditions under which the standard lamp was calibrated by the Bureau of Standards were not reproduced. Therefore the absolute values recorded cannot be relied on until further tests are made on the lamp, or another standard lamp which has been calibrated on a 50 cycle a.c. supply is available. The calibration specified the total radiant-flux density for wavelengths 313·2 mμ. and shorter, and the relative intensities of the lines from 436 to 230 mμ. (See Table XII.) The supply was 60 cycle a.c., the 220-volt tap was used on the transformer and the primary voltage was 220·7. The lamp current was not specified, but was probably close to 2 amps. The conditions under which the lamp was operated in our work were 50 cycle a.c., 230-volt tap on transformer, primary voltage 220, primary current 4·05 amps., lamp current 2·80 amps. The lamp was allowed 30 minutes to attain equilibrium temperature before readings were made.

Table XII.
Wavelength in mμ 436 405 365 334 313 302 297 289 280 275
Relative Intensity 39.5 24.2 48.1 4.0 35.7 12.5 7.4 3.0 4.6 1.9
Wavelength in mμ 270 265 258 254 248 240 238 236 230
Relative Intensity 2.0 9.9 2.6 16.9 3.3 1.5 1.7 1.1 0.4

The absolute energy of wavelengths 313·2 mμ. and less at 61 cms. distant emitted by the standard lamp was measured by the balanced thermopile method. The filter factor for the barium-flint filter was found to be 1·21 by the method described in Part I, and a re-determination of the mean thermopile sensitivity gave the result 2·02

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microwatts per cm.2 per μV., in close agreement with the value obtained a year earlier. The mean e.m.f. generated by the u.v. radiation excluded by the barium-flint filter, after correction with the Noviol A filter, was 41·6 μV. Hence the radiant-flux density at 61·0 cms. from the quartz envelope of the standard lamp = 41·6 × 1·21 × 2·02 = 101·7 microwatts per cm.2 in the wavelength region 313·2 mμ. and less. This figure was checked by repeating the measurements.

The procedure adopted in calibrating the p.e. amplifier bridge with the standard lamp was to eliminate wavelengths less than 275 mμ. by means of a calibrated Corex D filter placed over the p.e. cell. This reduced the uncertainty involved due to the presence of short waves with their high p.e. response and roughly limited the wavelength band to that required in solar radiation measurements. Then from the spectral energy distribution of the source and the transmission data of the Corex D filter, a conversion factor was obtained which enabled the absolute u.v. energy of wavelengths 313·2 mμ. and less falling on the cell to be calculated from the corresponding voltmeter readings. The data for the calculation of the conversion factor (P) are shown in Table XIII.

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Table XIII.
Wavelength in mμ. Relative Energy of Source. Transmission of Corex D. Relative Energy through Cx. D. Absolute Energy through Cx. D. Relative Response of p.e. cell. Effect on p.e. cell.
a b c d e f
334 4.0 86.7% 3.47 3.37 0.3 1.0
313 35.7 76.2 27.20 26.45 15.0 396.6
302 12.5 62.4 7.80 7.58 38.0 288.2
297 7.4 51.0 3.78 3.68 55.0 202.1
289 3.0 35.1 1.05 1.02 80.0 81.7
280 4.6 15.5 .71 .69 110.0 76.2
275 1.9 5.3 .10 .10 127.0 12.5

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Column c is the product of columns a and b. Column d is obtained by multiplying column c by the factor 101·7/104·5, since 101·7 microwatts per cm.2 is the absolute radiant-flux density for wavelengths 313·2 mμ. and less received by the thermopile, while from the emission of the standard lamp (Table XII) the relative units of radiant flux density in this region are 104·5. Column f is the product of columns d and e. The total of column f (1058·3) represents the energy flux as weighted (or appraised) by the p.e. cell when the output from the 5·2 cms. of mercury are exposed by the opening in the shield of the standard lamp is incident on it. But the effective are length was reduced by a diaphragm to 1·95 cms., which is approximately equal to the diameter of the aperture to the p.e. cell, so the actual energy flux weighted by the p.e. cell when 61 cms. from the lamp = (1·95 × 1058·3)/5·2 = 396·9. Under these conditions the voltmeter read ·714 volts when R6 was set at scale number 40, hence the conversion factor = 396·9/.714 = 556. Hence, weighted radiant-flux density = 556 × voltmeter reading (reduced to scale number 40). The absolute radiant-flux density then follows by multiplying the above by another factor (G), whose evaluation is discussed in the next section.