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In connection with the development of a physical photometer it was found necessary to measure voltages of the magnitude of 10 microvolts to an accu racy of 1 part in a thousand. The design and construction of a potentiometer of the desired range and accuracy are described. CONTENTS Page T. Introduction ____  43 1 IT. Potent iometer cireuii   _ 432 III , T hermal shield __ __      __ 432 IV. Causes of extraneous thermal electromotive forces in the measurement circuiL  ___ 433 V. Reduciion or elimination of extrane01ls thermal electromotive forceB-_ 433 1. Temperature differences in the space within the thermal shield_ 433 2. Temperature differences between the springs and contacts of the reversi n g key _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 434 3. Temperature differences between terminal leads of the fourterminal resistor  ___  434 4. Temperature differences caused by heat conducted into the thermal shield    __ ____ 435 (a) Along wires entering the shield __  435 (b) Alonl!; the actuating mechanism of the key __  __  435 VI. Other sources of extraneous electromotive forces and their reduction or elimination ___ ___ ____    __ ___ 435 1. In the unknown-voltage ci rcuiL  435 2. In the galvanometer circuit   436 VII. Adjustment of the reversing key __  438 VIII. Tests and performance _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 438

A potentiometer for measuring voltages of 10 microvolts to an accuracy of 0.01 microvolt