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A Simple and Inexpensive Integrating Photometer

1964; Oxford University Press; Volume: 15; Issue: 1 Linguagem: Inglês

10.1093/jxb/15.1.187

1460-2431

Marianne Powell, O. V. S. HEATH,

Advanced Chemical Sensor Technologies

Integrating photometers have been described by a number of authors (Blackman et al., 1953; McDermott and Gordon-Smith, 1951) but these instruments are somewhat elaborate. We have felt the need for a meter so cheap and simple that a number can be used in a single experiment, especially in those performed in glasshouses (with and without shading treatments) where considerable differences in light intensity can occur in space and time, not only between but also within treat ments. The vacuum emission type of photocell, as used by Blackman and Black, has the disadvantage of needing a stable external power supply. Barrier layer cells are free from this disadvantage but have (1) a very small output; (2) a response which is only linear with light intensity over a small range, at most up to a few hundred foot-candles; (3) a sensitivity curve somewhat like that of the human eye with greatest sensitivity to green light (which is of least use to the plant) ; and (4) an alleged propensity for ageing rapidly in continuous use. The first disadvantage is less serious if output is integrated over long periods (e.g. over a week) ; the second and third have been largely overcome, in the instrument here described, by the use of suitable filters ; the fourth has not yet occurred in our experience but a routine of periodic checks is desirable. We have used only one type of cell, the Megatron type B photovoltaic cell. The simplest way of obtaining an integrated measure of the current passing through a circuit is by means of a coulombmeter. This principle was used in the Siemens-Megatron meter in which the gas evolved in an electrolytic cell was collected and measured in a capillary tube. The present instrument employs a simple voltameter with detachable electrodes {E+ and E_, Fig. 1); periodically these are washed with distilled water, dried in an oven at ioo° C., and weighed to the nearest 0-2 mg. Some difficulty was experienced in finding a suitable electrolytic cell, the essentials being a low resistance and a response to minute currents at low voltage over long periods without masking by side reactions. Of the metals tried, silver has a high electrochemical equivalent but unfortunately at very low current densities the deposit formed on the cathode is of a dendroid nature ; it grows rapidly towards and makes contact with the anode, or drops off and must be collected for weighing. Owing to the nature of the deposit the same electrode