Abstract
Present industrial methods of photoelectric measurement are incapable of approaching the theoretical sensitivity limit set by thermal agitation noise because of various practical limitations restricting the maximum useful gain. A new method is described for converting the d.c. space currents in photo-tubes into a.c. voltages capable of amplification to the point where only fluctuation noise limits the accuracy. This conversion consists in the magnetic deflection or suppression of space currents with the use of an alternating magnetic modulating field applied transversely across the path of photoelectrons. A practical circuit is described with which photo-currents were measured close to the threshhold of agitation noise without use of other than commercial radio components. The new method affords complete separation of the actual photo-currents from all leakage currents in the input circuit without the use of a.c. light sources or light-chopping mechanisms.
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