RECENT METHODS AND TECHNIQUE OF PHOTOELECTRIC PHOTOMETRY

Abstract

During the last ten years a number of changes have been made in the methods of electrical photometry. True, the fundamental principles involved are still the same; the art depends upon the current generated by a photoelectric cell for a given amount of incident light, but the means of measuring this current have undergone considerable improvement. The photocell is an electrical device having an extremely high internal electrical resistance; the natural result is that the current generated by the cell is small, and the electrical circuit of the cell itself, loosely connected to the rest of the circuit as it is, is of high sensitivity and therefore easily influenced by variations in surrounding conditions. Ten years ago the photocurrent was not measured directly; instead, a rate of accumulation of charge was measured by means of timing the deflection of a sensitive electrometer. This method had several drawbacks which have been rectified by the introduction of the amplifying tube in 1932. The electrometer is, mechanically, a very frail and sensitive instrument, not well adapted to semiportable operation on the end of a telescope. The natural result was that under such unfavorable conditions, a special rugged type of electrometer was employed, and theoretical limits of sensitivity and stability were not attained. In addition, the fact that the electrometer depended upon the air for its damping effect made it necessary to operate the entire sensitive portion of the electrical circuit in this somewhat subversive medium. Even when care was taken to insure dryness of the air by some artificial means, the performance of the photometer could not always be depended upon, and photometers sometimes behaved in strange and quite exasperating ways. The introduction of the low-grid-current Pliotron type of vacuum tube made it possible to remove the mechanically delicate parts of the equipment from the telescope itself, and furthermore made practical the removal of air from the vicinity of the sensitive photocell circuit. The use of direct-current amplification with photocell and ampli-