An A-C resistance thermometer bridge

A 400 Hz brid ge des igned spec ifi cally fo r meas urin g the res ista nces of platinum th e rmom e te rs is desc ribed. Wh en use d in co njun c ti on with a co nve nt ional 25 ohm thermome ter, th e in strument ca n resolve a resistance cha nge of 2 mi cro-ohm s or less, or about 20 mi crodegrees. It is believed to be in e rror by no more th a n 3 mi c ro-ohm s througho ut th e usef ul temp e rature ra n ge of a con ve ntional th e rmome te r. Th e instrum e nt fea tures an a utomati c phase a ng le balance, and ex te nsive use is made of multi sta ge tran sform ers and operational amplifi ers.