The amount of liquid is regulated so as to reach the same height in the two branches when the bulbs are at the same temperature and zero is indicated on the two scales. A difference in temperature between the two bulbs raises the liquid above zero in one branch and lowers it by the same amount in the other branch. The scales on the two branches are graduated from 0 to 20 going downwards and from 0 to 50 going upwards.
The fact that the two bulbs are blackened indicates that this instrument was used in research on radio energy.