Excerpt from Instruments and Methods Used in Radiometry In the course of the discussion it will be noticed, as was pre viously known in a general way, that each instrument has some quality which makes it useful for particular kinds Of work. For measuring very narrow emission lines and determining dispersion curves the bolometer is no doubt the best instrument. For measurements requiring a larger receiving surface the linear thermopile is the more sensitive and the more precise. It has the further advantage that there is no permanent current. On the contrary, the bolometer has a current which heats the bolometer strips above the temperature of the surrounding air. This causes air currents which make the zero of the galvanometer unstable. This is not true of the thermopile. Less is known concerning the radiometer, which rivals the bolometer and the thermopile in sensitiveness. Furthermore, the radiometer is not subject to magnetic perturbations. Its window limits its useful ness to the region of the spectrum up to 20 u. The fact that it is not portable is a minor Objection. An attempt is made in the present paper to discuss all the important details involved in radiometry, so that it will be pos sible to gain a knowledge of the subject without searching through the already extensive literature. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully; any imperfections that remain are intentionally left to preserve the state of such historical works.