Self‐balancing resistance bridge

An automatically balanced resistance bridge with below‐picowatt dissipation in the measured resistance has been developed. The bridge, capable of measuring resistances between 20 Ω and 10 kΩ with an accuracy of 5%, was specifically designed to allow resistance measurements to be made at low temperatures without introducing an appreciable heat source to the cryostat. It consists of a Wheatstone bridge having a photoresistor as one of its arms. The feedback balance is accomplished by means of light‐emitting diodes which control the photoresistor. A novel feedback principle used in the bridge is described.