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As a result the total equivalent noise resistance of the meter (including the noise in the radiofrequency amplifier) reduced to a temperature T = 4.2 ยฐ K, at the input to the transformer is ~20 fL This was checked with calibrated resistances of different magnitudes, connected to the transformer input directly in the cryostat. The results of these measurements are shown in Figure 1. The abscissa gives the resistance referred to room temperature and to the input of the radiofrequency amplifier R* = Rk~rT/293 and the ordinate gives the square of the voltage at the amplifier output. Curve 1 corresponds to connecting the calibrating resistance directly to the amplifier input (T = 293 ยฐ K), curves 2, 3, and 4 to connecting the calibrating resistances to the transformer input (the input stage and the resistances were at 4.2, 77, and 293 ยฐ K). From these curves it is seen that with the input stage described the instrument can measure thermal noise e.m.f.s of an ~20 f~ resistance at 4-2 ยฐ K or a sinusoidal e.m.f, of between approximately 5 and 7 x 10 -~t V. We note that resistances of ~-,0.3 f~ at room temperature have this kind of thermal noise e.m.f.
The instrument described can also find application in measuring small e.m.f.s of ~10 -Iยฐ V from low impedance sources at "room temperature, using the appropriate arrangement of leads in the cryostat. Yu. V. Lavrent'ev and V. P. Maiorov took part in assembling the meter. The authors are grateful to M. G. Golubtsov for providing the low noise radiofrequency amplifier.
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